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Are all humans in Naruto universe mandated to admit in the shinobi academy?
All humans in Narutoverse have chakra and that anyone can be a shinobi if anyone is able to use and manipulate chakra to a greater effect.

I always thought that everyone born are admitted in the academy and there, they learn how to use their chakra and rise through the ranks and become a ninja similar to schools and education in real life. The Academy students were trained and prepared for life as a shinobi. Is this true? Is it mandatory for all humans in Narutoverse to admit in the academy and be a ninja?
We see quite some non-shinobi professions, for e.g.Teuchi becoming a Ramen chef. Just like my above assumption, did he once admitted in the academy in the childhood but choose to become a ramen chef?
Is it mandatory to for all to take admission in the academy and learn the basic elementary jutsus and experience the initial life of a ninja before becoming something else?

Is it mandatory to for all to take admission in the academy and learn the basic elementary jutsus and experience the initial life of a ninja before becoming something else?

Most probably yes. Everyone must know the basic jutsus. Then, it's upto them to clear the exams to be a full-time shinobi or quit the academy to be something else.

We see quite some non-shinobi professions, for e.g. Teuchi becoming a
Ramen chef. [...] did he once admitted in the academy in the childhood
but choose to become a ramen chef?

YES. He would have known basic jutsus but didn't progressed that far in the Shinobi world. Maybe he was a pre-genin academy student or had acquired genin stature before quitting. Since he is a chef, he might be skilled in Kenjutsu and talented with Kunai and shuriken. He would probably possess an Earth and Water Release affinity, possibly Fire Release but the Chakra Reserves was probably averaged sized and ultimately run out of chakra fairly easily. He doesn't know a whole arsenal of jutsu but knows quite a few of them.
Now, an image circulated quite a bit which spread the fact that Teuchi might have been taught by Jiraiya but it was never confirmed.

Teuchi could've been the team member of Jiraiya team-7 since they would be around the same age group and it would explain why Teuchi wasn't afraid to scold Shinobi for nonpayment, because he was a retired Shinobi himself. It would also explain why he never shared the same resentment as the rest of the village towards Naruto as a kid and genuinely accepted him, giving him free food sometimes and treating Naruto as a valued customer. He does this because he was teammates with Naruto's father and knows that Naruto is Minato’s son. He can see the resemblance in Naruto and knows something great will come of him. This would also explain the respect that he has towards Jiraiya.
Source: https://www.quora.com/If-the-ramen-guy-from-Naruto-was-ever-a-shinobi-in-his-youth-what-kind-of-jutsu-would-he-have-and-why

It's voluntary some families seemed to push their kids into being Shinobi having had certain powers passed down through their bloodline which lead them to being conduits for Government officials and the mass majority seem to want to join the academy because it's a prerequisite to becoming Shinobi which are highly respected and centre of all awe.


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Approximately when is the story of "Boku no Hero Academia" set?
As nearly as I can tell, the events of "Boku no Hero Academia" seem to begin some time around 2174.  I base this estimation upon the manga having been begun in 2014 and the current All Might being the 8th such hero (and taking an assumed average of 20 years per generation between passings-on of their power).
Is there a definitive answer for this?  If not, are there canon facts, observed trends, or even fan-based assumptions that might at least weigh to the contrary (whether earlier or later than 2174)?

There doesn't seem to be a defnitive answer. But in regard of fan theories, Google came up with this and this.

It's around the years 2060 and 2085. Deku's hero license says 20XX on it, so it can't be past 2099.
1st gen All Might is around 1950. Also, tech has progressed because of quirks.

Please include relevant sources/references to support your answer.

@Cak3 I believe that your point is valid: upon Googling "Deku's hero license" I found some fairly clear copies, and it does list an issuance date of "交付 20XX".

I've been looking around on different sites to get a clearer answer on this, but the most I can tell is that at least the current timeline of Deku at UA takes place in the 21st century since his ID apparently reads 20XX. This is common in a lot of anime/manga from my experience, but if we take that as an actual indicator, then the latest it could possibly take place is 2099.
Now, if you want to start mixing fandoms, I do recall that the manga makes reference to Marvel characters, and if we mix the MCU into things, then MHA would be taking place not too far from our own timeline.

Unless canon ever gives us an exact year, it’s impossible to say for sure. Thematically though, the story takes place 20 minutes into the future.
Technology and culture is almost identical to today, except that there are people with superpowers and this has been for generations. The problems that the character’s face, and society as a whole face, aren’t that different from the problems we face in real life (except dialed up to 11 because of the presence of Quirks, and the fact that this is an action work of fiction).

First of all, there's the year that the glowing baby was born. There's no way of knowing what year it was, but since technology isn't that much more advanced than ours, and we're told it was halted by the appearance of quirks, we can safely assume that it was in the early-to-mid 21st century. Then there's the fact that we see snow in Japan, a near-equator country getting hit by warmer winds from the ocean. Since dealing with quirks tore the world's focus away from other issues such as climate change, I think we should definitely assume that the world had time to deal with humanity's massive carbon footprint, something that, given our current situation, would likely take twenty or so years to properly deal with. Yes, I know there's the horribly trashed beach that All Might had Izuku clean up, but litter and greenhouse gases are two different things, and given the current real-life situation, I think it's safe to say which one the world prioritized. Besides, a lot of that could easily have been floating in the ocean for decades. All in all, I think the glowing baby was born around 2050.
Then there's the initial backlash and the appearance of All for One. It likely took the world between five and ten years to fully react to the initial appearance of quirks, and All for One appeared pretty early on, so we'll say that All for One appeared about ten years after the glowing baby, around 2060.
Now for how long the quirk society has been around. The doctor specifically states that Izuku is a part of the fourth generation. In the developed world, parents tend to have kids near middle age, so I think it can be safely assumed that each generation is around thirty to forty years (we'll round it at thirty-five on average). If Izuku was born early in his generation, we can probably say that he was born around 110 years after the fiasco with All for One and his brother, putting us at around 2170 when he was born. This checks out with All for One having been in power for over a hundred years.
Now, one reason you may think it doesn't check out is the fact that Izuku is the ninth holder of One for All, something we previously said appeared around 2060. There are those who draw a pattern from the fact that Toshinori and Izuku both received One for All at the age of fifteen, and saying this probably went for everyone. However, they are special cases due to them both being quirkless kids in a superpowered society that wanted to go to a high school for heroes - something that kind of required a quirk. Most of the bearers were likely adults when they received it, due to experience as a hero and with the quirk they already had being a good mix with something so powerful. They likely received it at a later age, shortening the time they had to use it. Most notably, All for One's brother probably didn't hold on to it for very long, as he knew early on that he would need to pass the quirk on to future generations. With All Might having probably had the quirk for around fifteen years before Izuku was born, and the first user probably holding it on for ten to fifteen years, this puts the average time of holding the quirk at fifteen years for the others, though some may have passed it on prematurely due to injuries or expecting to do something that would get them killed, so I think it's safe to assume that, in this timeframe, the median retention time of One for All was around twenty or so years.
So with all of that, in addition to Izuku being fourteen at the beginning, fifteen at the current point in the anime, and sixteen at the current point in the manga, I believe that My Hero Academia takes place right around the turn of the 23rd century.

Japan's not terribly near to the equator (it's ~31° N through ~45° N latitude), and snow is reasonably normal there (I spent 4 years in Tokyo), but the rest of your reasoning is sound.  Your resulting 2170 is much like my estimated 2174, which reassures my own guesses, since you took a different route to arrive at that (I particularly like the observation about All For One's reign being >100 years).

Calculating the time, if Endeavor was born in 1989, it would be 2035 since he's 46 in the current My Hero universe. But considering this is also in the past and it's the story being told by Deku, it might be 2048 and by the end, it would be 2134.

It takes place in our century but in a different universe where 80% of people have powers. I said our century because the technology is the same as ours and the different universe because of the power.

I think it's set into a decent future.
From what I've seen (which is mid-season 2), I'm not totally sure and I came here to find this out, but I actually don't think it is just 20 minutes into the future as Mr F.'s answer mentioned.
I mainly came here because of a quote stating that it is predicted that humans would've been a lot more advanced if not for the development of quirks slowing everything down and how we would have been expected to have had space cruises by now, so I expect it's around the year 2100.

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Is there any significance to the "shiki" most Otsutsuki have in their names?
Well... the title says it all!
Most of the Otsutsuki clan members, if not all to appear in Boruto have "shiki" at the end of their names. Examples would be Momoshiki Otsutsuki, Kinshiki Otsutsuki, Urashiki Otsutsuki (filler), and Isshiki Otsutsuki.  There are indeed some Otsutsuki without shiki in their names, such as Ashura, Indra, Kaguya, Hagoromo, Hamura, and more. However it raised my eyebrow when almost all Otsutsuki in Boruto had "shiki" in the end of their name, so I wondered if it has any significance.
So, is there any significance to the "shiki" most Otsutsuki have in their names? If so, what is it?

Not sure if related, however according to Google Translate from Japanese to English, “Shiki” is roughly translated to “Four Seasons”. Based on your question and wiki, I count a total of four Otsutsuki members (anime and manga) with the suffix “Shiki”. So I’m wondering if each one represents a season (Sping, Summer, Fall, and Winter)?

Ah, makes sense. I also went to Google Translate to find the meaning of shiki, translating to four seasons, however Japanese translate is very inaccurate there so I came here for the question.

The characters mentioned are all based on Japanese folklore. The original names of the folklore characters are Momotaro, Kintaro, Urashima Taro, and Issun Kotaro. -Taro is just a common way to end boy's names, similar to -maru. In Naruto, the characters were named by taking the first two syllables and add -shiki to the end. The exact significance of -shiki in Naruto is still unknown, if there is any (There are several meanings of -shiki in Japanese depending on context, but I don't what is the context in Naruto). Some of the Otsutsuki mythological references are pretty obscure, so it is hard to know.
Also, there is also a theory proposed by a redditor that the -shiki suffix us given only to the main members of the Otsutsuki clan.
Source:

https://www.reddit.com/r/Boruto/comments/ich72h/why_does_almost_every_otsusuki_has_shiki_in_their/
https://www.reddit.com/r/Naruto/comments/e0an6d/theory_the_shiki_names_are_only_given_to_members/

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Is Hagu's name a reference to The Hague?
In Honey and Clover the character Hagumi Hanamoto often goes by the abbreviation of her name, Hagu. I just noticed this is spelled similarly to The Hague, a city and municipality in the Netherlands. In the story, Hagu is a genius artist, and a precursory internet search shows that The Hague does have connections to art. For example, from wikipedia:

The Hague has its share of museums, most notably the Mauritshuis, located next to the Binnenhof, which exhibits many paintings by Dutch masters, such as Johannes Vermeer, Rembrandt van Rijn and Paulus Potter. Other museums include the science museum Museon, the modern art museum Kunstmuseum, the historic museum Haags Historisch Museum, the national postal museum Museum voor Communicatie, the Museum Bredius, the Louis Couperus Museum, the museum Beelden aan Zee in Scheveningen, and the Gevangenpoort, a former prison housed in a 15th-century gatehouse.

It might just be coincidence, as Hagu and Hague aren't pronounced similarly. On the other hand, checking the Japanese wikipedia page for The Hague, it seems the pronunciations are more similar in Japanese, at least (ハグ vs ハーグ). It would be interesting if the author had this connection in mind. Is there any evidence for this being a reference?

While it is difficult to prove this with any certainty, we can try to analyse the facts around it to make an educated guess.
First, as far as I can find, the author of the manga has never said anything proving or disproving this relation, which unfortunately does rule out any definitive answer in either direction.
Do note that she has mentioned that the title of Honey and Clover comes from the album titles of the musicians Shikao Suga and Spitz, which can be seen as indication that she does make references.
However, the name Hagumi, and in turn the nickname Hagu, is common enough that inferring any significance from this alone is difficult.
A prime example from anime would be Hagumi Kitazawa from Bang Dream!, who is referred quite frequently as Hagu externally (and within the source material IIRC)
This is personal opinion, but if the author wanted to make a reference via character name, it seems much more likely that she would have used the name of a famous artist (perhaps a Japanese one, to avoid any finagling to form a plausible Japanese name) than The Hague, whose claim to international fame as an artistic city is frankly debatable.
The combination of a commonplace Japanese name and a tenuous artistic association to "reference" makes it, in my view, much more likely that this is a simple coincidence due to the sheer volume of names with some artistic significance.
Of course, without a clear statement from the author, all this is necessarily mere speculation.


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If it is Otsutsuki law not to kill another member of the clan, why are other Otsutsuki sacrificed as Ten-Tails fodder?

We know now that Otsutsuki members CANNOT kill other members without the permission from leader of the clan. (source)

Otsutsuki members cannot kill others from their clan without permission from the leader, yet they are doing just that each time they feed an Otsutsuki to the Ten-Tails in order to cultiavate a Divine Tree.
So why is this allowed? Killing another Otsutsuki isn't allowed.

This was done in order to make sure the Divine Tree was planted and the Chakra Fruit bore.  And despite how crazy some Otsutsuki can be, I highly doubt that they would allow one to do something that could wipe out an entire planet without consulting the clan's leader. In fact, the main reason Otsutsuki operated in pairs (e.g Momoshiki-Kinshiki, Isshiki-Kaguya) was in order to sacrifice the lesser rank, in this case, Kinshiki and Kaguya, as shown in Boruto Chapter 51:

As previously mentioned, the Divine Tree isn't a toy and isn't something anybody could be entrusted with--which would make it illogical for this to be done without consulting the leader. The superior (Momoshiki, Isshiki) devours the Chakra Fruit, and the subordinate (Kinshiki, Kaguya) is sacrificed for the tree to be harbored.
As explained by Amado, they have the Karma seal to give onto someone. So if all goes well, both Otsutsuki are alive and the plan to eat the chakra fruit is completed. The leader (yet to be shown) would obviously know this process to plant the Divine Tree, giving more reason they would know the sacrifice needed, which grants permission for the subordinate to be killed.


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Why Light didn't lose memory in ep31
In ep31, Light says he had misa sign off as the notes owner, and hence she loses her memory. In the earlier eps, it was shown that Light used misa's note to keep his memories. But as soon as the book was given away, why didn't he lose his memories?

When Misa resigns his notebook she loses his memory and the ownership is transferred to Light (I guess). Besides, Light always kept a piece of paper from the death note.

This actually makes sense. But what happened when his father was in death bed? Were there 2 death notes to begin with? or did the 3rd shinigami take one back?

This is actually quite a math, but very simple:

Before ep27 there was 3 Death Notes in the human world - one of the police (original Light/Ryuk's Death Note) (it's actually the investigation unit but I will write police for convenience), second of Light (was original of Rem), and third was misa (original brought to her by Rem).

By the events of ep29 the police Death Note had been traded to Mello's mafia.

In ep30, Light took Misa's Death Note in his care so he won't forget, and sent his currently Death Note to the police, so his father would use it to get the Shinigami eyes from Ryuk. ~ Currently in this phase, Misa is own the Death Note Light holds, so while he gave up his Death Note, Misa is the owner and Light is the borrower so he won't forget; In other words: Death Note #1 - Mellow, Death Note #2 - The police, Death Note #3 - Misa.

By the end of ep30, the police took back Mello's Death Note - the third DN brought back to it's original Shinigami. The Police Death Note that Shuiciru - Light's father holds and its owner, is dead, while Light holds the Death Note of the police, meaning he's now the new owner of this Death Note, while this DN is held by the Police for the rest of the series. Light doesn't need Misa's DN for his memories and gives it back to her.

In the Near arc, he made Misa forfeit her DN and give it to Mikami, becoming normal girl without DN memories. Light memories got nothing to do with it since he owns the police DN.

All this also explains the reason why Mikami couldn't see Light's Lifespan in the latest episodes - he was the owner of the Police DN, and didn't need to actually hold it to regain his memories.

For the same reason he was able to defeate Yotsuba Kira: When he touches a note that he once owned, he temporarely regains/ keeps his memories.
I don't find a reference just now but some time after the first major climax half way through the manga, Light explains to Ryuk/ us that he keeps physical contact to a note using a corset. As long as he touches a note he once owned (and at some point he owned every last one that became part of the human world at some point), he temporarely holds his memories. And since it is old trickster Light, temporarely happens to be several months.
I will go through the manga again late this week and hope I can find the chepter.


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In I'm a Spider, So What, is Sophia Keren the baby vampire the same as the adult vampire serving the demon lord?
In the anime, latter half of the first season, we see that Kumoko saves a baby human vampire from some bandits and later guards her by camping out near the town. We also know that this baby vampire is a reincarnation. (I believe it's Negishi Shouko)
There's an adult Sophia vampire that serves the Demon Lord Ariel. We also find out she's a reincarnation. I thought these were both one and the same, just huge time gaps, but in episode 20 we see adult Sophia heading to attack the town where baby Sophia is, and from what I can tell it's in the same timeline. Are the two Sophia's genuinely different people, connected in some way, or are they the same person and I'm misunderstanding the timelines?

Yes both are the same person.
Your confusion comes because in episode 20 there is a shift in time that is not super obvious. There was a short transition scene after Kumoko talked to herself about what she was going to do regarding the army heading to Keran County of the Sariella Country. After she made up her mind and left the Tree, it showed the same tree several times but as time passed, going from a thriving tree full of leaves to a seemingly dead tree with no leaves indicating that time had passed. This transition is easily missed. The Army with Sophia, Hugo, and the Demon lord is heading towards the Elf village.


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TV Saint Seiya Prologue theme
In Sanctuary chapter, after opening, there is a prologue for the events of the last episode. I can't find the name of that music theme, and "Galaxian Wars" is not that I'm looking for.

I think this is the one you are looking for: Intense! Cosmo

It should be "Galaxian Wars" composed by Seiji Yokoyama. You can easily find it on Youtube. Try make sure of it.

Is another one that I'm looking for. Galaxian Wars is played mostly but, I'm searching for the one is played at the first episodes (Phoenix Saga, Silver Saints)

Ah, sorry it is "Intence! Cosmos". Try youtube. My friend uploaded the video.

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Is there a name for Sailor Moon-style transformations?
Sailor Moon (and friends) have a recognisable transformation style. Other anime—like Dragon Ball and Puzzle & Dragons X—parody or (likely) take inspiration from such transformations.
Is there a name for this style of transformation? It’s so recognisable that it feels like it should, yet I’m not finding anything.
Bonus: Did this style of transformation originate with Sailor Moon, or did it just popularise it despite there being prior art?

Yes, there is.
This is called a henshin shiin (変身シーン), which means "transformation scene."
The magical devices that characters use to accomplish them, such as Sailor Moon's brooches or the other sailor senshi's pens or sticks, are all called henshin aitemu (変身アイテム), meaning "transformation items."
A few related terms are henshin poozu (変身ポーズ), meaning "transformation pose" - that is, the final pose the character strikes when the transformation sequence is finished - and henshin no BGM (返信のBGM) or henshin kyoku (変身曲), meaning "transformation background music" or "transformation song" (although each of these musical pieces have their own title, such as『ムーン・プリズム・パワー・メイクアップ！』(Moon Prism Power, Make Up!), 『ウラヌス、そして、ネプチューン』(Uranus, and also, Neptune), or 『スターパワーメイクアップ！』(Star Power, Make Up!).
This style of transformation scene existed in the long-standing mahou shoujo (魔法少女: magical girl) genre that falls within the broader shoujo (targeted at the demographic of young girls) genre long before Bishoujo Senshi Sailor Moon came into existence in the 1990s. Magical girl manga dating from before Sailor Moon include panels representing it as well, but the primary reason for these extended scenes in traditional cel animation was to use up a good amount of time in each episode of higher quality, reusable cels that could reduce the amount of other cels needing to be hand-drawn to come out to about a 22-minute-long run time. Arrival scenes (where characters recite a trademark phrase) and attack scenes serve the same purpose. After the switch to computer animation, this time-saving repetition became less necessary from an animation company's production standpoint but, because such scenes had become iconic over the past decades' worth of magical girl series, they are still used in fully-computer animated series, such as Pretty Cure - and they still continue to serve the same purpose (to reduce the workload of the animators per episode), albeit if less crucial. Ironically, in many cases, these more recent scenes have stretched out considerably longer than they had ever been in the classic magical girl series (perhaps to poke fun at the genre, perhaps to show off what computer animation can do... perhaps both), even as the practical need for them has largely decreased.
What Sailor Moon did innovate was combining that decades-old genre with the sentai (戦隊: team-fighting) genre - after Sailor Moon, many other series copied this, leading many people without knowledge of the classic magical girls to assume that the genre is usually comprised of teams of superhero girls, but that was not the original style, in which either a) a single girl from another magical world came to this world, or b) a single ordinary girl in our world suddenly received magical powers (in most cases, to use for nichijo [日常: daily life] situations rather than for saving the world from monster or alien villains). Nor have all magical girls since Sailor Moon employed teams and extended transformation sequences.
For more details, see "Have the creators of Tokyo Mew Mew or Sailor Moon commented on the similarities?", "Why is “Puella Magi” equivalent to “Mahou Shoujo”?" and "Why do the senshi get a manicure before they get gloves?"


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If Itachi wanted the best for Sasuke, why did he ask him to kill his best friend?
Supposedly everything Itachi did was for the best of Sasuke. To keep him alive, to give him a purpose to survive and get stronger, etc. But he also asked him to kill his best friend to get the Mangekyou Sharingan. If he wanted the best for Sasuke, why would he ask him to kill his best friend, when this doesnt seem to have any apparent purpose for the best of Sasuke, and instead the idea would traumatize him more?

Maybe so Sasuke could get the power he wanted to kill Itachi? Not sure. Will have to research. Itachi pretty much lived to die at Sasuke's hand.

There's need to be clarification - the mangekyo sharingan isn't obtained by killing your best friend but seeing him dying.

@USerNAme quite both, actually. Killing your friend would make you see him die. During Madara's time, Uchiha clansmen slaughtered eachother in hopes of obtaining the MS (which they did succeed in) and other cases, like seeing Shisui die, the MS is also awakened.

@ᴅᴀɴɪᴇʟッ - kakashi's / obito's sharingan awakened because of seeing friends dying, not because killing.

Yes, the most common case is by SEEING the friend die. But as I pointed out, once many Uchiha clansmen figured out how the MS is awakened (via Madara & Izuna) they killed other clansmen and still got the MS.

First Theory
Considering the end result was Sasuke rejecting Mangekyō, Itachi may have told Sasuke how to get the Mangekyō Sharingan in an attempt to bias Sasuke against it. Itachi has been shown to be a very perceptive and conniving person. Examples of this are abundant:

He sets a trap inside of Naruto to use Shisui's Sharingan to
reprogram Sasuke to defend the Leaf Village. He ultimately ended up
using it to reprogram himself while reanimated.
He set a trap inside of Sasuke's eye to incinerate Tobi should the two cross paths.
His interaction with Sasuke after slaughtering the Uchiha Clan was done to manipulate Sasuke into eventually killing him, with the goal of making Sasuke a hero of the Leaf Village.
In the fight with Kabuto, Itachi was quick to discern how Kabuto's nature as a spy was similar to his own. And then he set a trap by manipulating Kabuto into performing certain actions, so he could use the Izanami, which ensnared Kabuto in a time loop until he became the person Itachi wanted him to be.
He threatened Danzo in order to ensure Sasuke's safety. He obviously understood Danzo well enough to know this would keep Sasuke safe. After the Third Hokage's death, he specifically made an appearance to remind Danzo of their arrangement.

Seeing all this, it isn't entirely far-fetched to suppose Itachi may have foreseen the outcome of telling Sasuke about the Mangekyō Sharingan. And the outcome was indeed that Sasuke rebelled, disgusted by the idea of gaining power the same way as his brother, ultimately choosing to leave Naruto alive.
But why might Itachi do this instead of, for instance, just leaving Sasuke in the dark about this secret? He may have seen it as an inevitability that Sasuke would find the Uchiha Clan's secret stone tablet, in which case he had to do something to make Sasuke not want to try and get the Mangekyō. Well, when someone you hate tells you to do something, it tends to make you not want to do it.
This is just some speculation, and I don't necessarily believe it myself, but I thought it was worth delving into as an alternative to the other answers.
Alternate Theory
As an alternative to what I said, I'd first like to point out that not everything Itachi did was motivated by his desire to help Sasuke. If that were the case, he never would have slaughtered the Uchiha clan, which was a seriously traumatic event that Sasuke was never going to get over (and indeed, this one event basically colored Sasuke's entire life experience).
In that case, it is prudent to note that Itachi was also motivated by a desire for stability in the Leaf Village. In order to do that, for there to be no repercussions against the Leaf Village when he carries out his mission, he had to be the bad guy. And in that case, he has to make his motives seem believable. Thus, he tells Sasuke about Mangekyō not for Sasuke's benefit, but as a clever cover, as if to say, "By the way, do you want to know why I killed Shisui?"
Itachi's stated goal was to make himself the villain, and Sasuke the hero for defeating him. Sasuke's not going to be a hero in the Leaf Village if he kills his best friend (who would presumably be a Leaf Shinobi). This further suggests my first theory might be correct (these two theories aren't mutually exclusive, as Itachi may have multiple motives for doing things).
Again, Itachi had to be the villain. Thus, Itachi and the Leaf higher ups left a lot of evidence that he was the bad guy, for the other Shinobi, and for Sasuke. Danzo, for instance, upon arriving at the scene of the massacre, immediately declared that Itachi secretly harbored a deep hatred for his clan. And they did a good job because the result was that no one ever suspected a thing. Even Anbu like Kakashi, who'd been helping spy on the Uchiha and knew about their discontent, and who personally knew Itachi, never thought that Itachi was under orders. A shocked Kakashi instead concluded that even after all that time working together, he'd never really known Itachi.
Additionally, we have to remember Itachi is playing a role. If we're thinking on the role of villain Itachi, with his motives, it makes sense for him to tell Sasuke about Mangekyō. This though it seems it isn't necessary for Eternal Mangekyō, it still seems like it would have been preferable. In which case, Itachi, acting out his role, wants Sasuke to get Mangekyō. And I'll note that having a secret motive adds plausibility for everything Itachi did, so it's good he didn't just tell Sasuke, "Hey, I want you to get Mangekyō so I can take your eyes and make myself more powerful," but instead, revealed it to him later. Because at that point, Sasuke would be like, "I'm older now, and I have questions about the Uchiha massacre." But hearing Itachi wants his eyes, he'd be like, "Oh, now I get it."
Third Theory
It's possible that Itachi wished for Sasuke to become stronger. Perhaps because of the danger Danzo posed, he wanted Sasuke to have the Mangekyō. He may have felt it was necessary for Sasuke to have Mangekyō to defend himself. Or more likely in my opinion, he simply wanted Sasuke to be informed of the possibility. Being a spy and confused about what's right, Itachi may have been uncertain whether or not Sasuke should get Mangekyō. He wanted Sasuke to decide for himself. In which case, he told Sasuke while thinking something like this, "Well, Sasuke, I don't know what you should do, but if you want power, here's how to get it."

An interesting perspective. +1. Assuming Itachi did know about the consequences of the mangekyou (going blind from overuse), I guess it'd make sense for him to attempt to pull that off and plan to wait till partially blind to battle sasuke and die.

As I mentioned in the comments:

Maybe so Sasuke could get the power he wanted to kill Itachi? Not sure. Will have to research. Itachi pretty much lived to die at Sasuke's hand. (emphasis mine)

Taken from the Naruto wiki article on the Mangekyo Sharingan:

A Mangekyō Sharingan is distinguished from a normal Sharingan by its appearance, which changes the form of the tomoe seal. While the exact design differs for each user, they all resemble pinwheels. It is initially awakened by the trauma suffered from witnessing the death of someone close to the user.

I also pointed out in the comments that this can also be awakened by killing a close friend. Many Uchiha during Madara's era purposely did this after hearing how this eye was awakened in order to see if they could get it themselves. This process was succesful, which proves the Mangekyo Sharingan can also be awakened by killing a close friend, aside from having to see them die.

 This fact is quite popular already, but it's getting spoiler tagged because it is far-ish in the series. Sasuke figured out that Itachi killed the Uchiha alongside Obito on the order of Konoha's elders. This was not intended.

Itachi wanted to die by Sasuke's hand. He said himself he deserved to be dealt with by another Uchiha. Even if he wanted the best for Sasuke, he still wanted him to kill his friend in order to give him the power of the Mangekyo, so Sasuke would be strong enough to kill him, as he always wanted to. This would fulfill Sasuke's long goal of revenge, with hopes of him not finding out about the Uchiha incident. To answer the question, although Itachi always wanted the best for Sasuke, he wanted him to kill his best friend to:

Inform him about the Mangekyo Sharingan and how to awaken it
Give him the power needed in order to get revenge, with Itachi intending for Sasuke not to learn about the Uchiha incident

Yet another example of Itachi being selfish to "help" Sasuke :D

Itachi knew the only way Sasuke would obtain the Mangekyō Sharignan would be by killing his best friend which would make Sasuke about 10-20 times stronger than he already is. Itachi wanted Sasuke to be strong enough to face any challenge in the cruel shinobi world they live in and he knew having the Mangekyō would give Sasuke a huge advantage which it obviously did. Shortly after attaining Itachi's Mangekyō Sharingan, Sasuke was even strong enough to take on the Raikage at such a young age.

You cannot assume Itachi wanted Sasuke to kill his best friend. All we know is that he told Sasuke how to get the MS and that is by experience the intense emotion of seeing someone you love die. This is ultimately how he did obtain it after watching Itachi die.
Itachi knew that a strong Sasuke was important for the future of Konaha. He also knew that there was literally no one else who could teach this to him. Seeing as when Kakashi shows Itachi that he also has an MS (although an incomplete one) Itachi is shocked and astounded. Noting that this is to be expected from a Konaha shinobi.


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When people talk about Princess Sapphire's male and female "hearts"... Is it possible it's supposed to be "souls"?
When people talk about Princess Sapphire's male and female "hearts"... Is it possible it's supposed to be "souls"?
Because it seems that memories are tied to her "hearts", given what I've heard about the effects when they're removed, changing her feelings on things, and perhaps her abilities too...
I'd think there should be at least another question on the site about how "hearts" and "souls" / "minds" are sorta mixed together in Japanese?
Perhaps this one, about the kanji... https://japanese.stackexchange.com/questions/6336/where-exactly-in-your-body-is-%e5%bf%83

心「こころ」(romaji: kokoro), according to jisho, means "mind; heart; spirit". So, it's not so much the physical heart as the methaphorical meaning.
The anatomical term is 心臓「しんぞう」(romaji: shinzou), for the record.
So I don't think it's really meant to be the physical heart but the conceptual "core of one's feelings and emotions" - sort of like when someone says someone is "heartless", it's not to mean that they literally lack the physical blood-pumping organ.


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Why does a similar cursed spirit manifest at Yasohachi Bridge as had manifested at the juvenile detention center?
In episodes 23 and 24 of Jujutsu Kaisen, we find out that one of the fingers of Ryomen Sukuna has been hiding its presence at Yasohachi Bridge since many years, and has only become active since Itadori Yuuji first consumed a finger a few months prior. In these two episodes, Fushiguro Megumi is forced to confront a special grade cursed spirit that manifested from the cursed energy of Sukuna's finger at Yasohachi Bridge.

What confuses me is that this special grade cursed spirit has the same appearance and personality as the one that metamorphosed from the curse womb at the juvenile detention center, which Yuuji, Megumi and Kugisaki Nobara investigated in episodes 4 and 5. A finger of Sukuna was responsible for manifesting that special grade cursed spirit, too.

Question: Is it the case that any cursed spirit that a finger of Sukuna brings into manifestation will have the same form? If not, how did the cursed spirits at the juvenile detention center and Yasohachi Bridge appear so similar (one might even say identical)?

sukuna needs it for his true form at 20 finger aka all 20 sukuna finger
for he take yuji itadoris body like in manga and anime

Is it the case that any cursed spirit that a finger of Sukuna brings into manifestation will have the same form?

Yes, they are called Finger Bearers. They are cursed spirit that becomes a special grade after they devour one of Sukuna's fingers. Finger Bearers begin as cursed wombs where they gestate until birth. During this period they can be seen by non-sorcerers. Once born they create an Innate Domain, a precursor to a Domain Expansion, that warps the dimensions of its birthplace.
Finger bearers appeared in both Cursed Womb Arc(ep 4-5) and Death Painting Arc(ep 23-24).


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How can Edward perform human transmutation on himself?
While he was trapped inside Gluttony, Edward performed human transmutation on himself in order to access his Gate. This means that he had to deconstruct and reconstruct himself, but I don't understand how he was able to re-create himself when he doesn't even have basic medical knowledge.

Afaik Edward and his brother did extensive research into the human body in order to prepare for the transmutation of their mother. The first step of transmutation is comprehension after all. I would say that counts as basic medical knowledge.

Hmm, but later in the series, Edward is unable to heal himself after he gets impaled.

Also see: https://anime.stackexchange.com/questions/8699/if-the-truth-didnt-exist-would-that-mean-that-feats-such-as-human-transmutat/8700

@alexgbelov Healing alchemy is very much different from normal alchemy and is called  alkahestry which is used in the country of Xing notably May Cheng.  Alkahestrists are capable of a higher level of medical transmutation than Amestrian alchemists - transmitting chi through the pathways of the human body to cure mild ailments as well as minor injuries. Amestrian alchemists are not expert in this field and they require a catalyst i.e. philosopher's stone to perform healing.

Alchemy is not a walk-in-the-park task. The process of learning alchemy is arduous. While some people are naturally talented and intellectual than others, perfecting alchemy takes years of practice. From a young age, Edward and Alphonse Elric were exposed to alchemy, especially as their father had been a brilliant alchemist and left a gargantuan collection of books behind for the kids to read. This, however, was in addition to their already curious minds and their prodigal level of skill.

[picture from this answer (please upvote if you like my answer)]

[...] he doesn't even have basic medical knowledge.

Wrong. Edward Elric was an alchemical prodigy and almost everyone considered him to be a genius. From wiki:

Edward Elric is an alchemical prodigy of genius level intellect. From early childhood, he displayed proficiency for alchemy, acquiring self-taught alchemy skills and comprehension from the advanced literature in his father's abandoned study at an age. Having incorporated alchemy into his life during his developmental stages, Edward sees the world alchemically and is, therefore, able to solve complicated codes, puzzles, and mysteries much more easily than normal. He has also shown himself to be exceptionally adept at chemistry and reasonably competent with biology; indeed even his preparations for the failed attempt to resurrect his mother were indicative of these. Edward's near-encyclopedic knowledge of alchemical theory is astounding and contributed to his incredible success with emergency Soul Binding experimentation. He shows an uncanny level of focus that allows him to effectively shut out everything from his surroundings and devote all of his considerable mental power to whatever problem demands his attention, especially in the case of books.

He did had a superior medical knowledge and hence attempted to resurrect his mother with the help of the laws of alchemy but due to limited experience it failed. But after years and years of study and practice and sheer perseverance, he gained knowledge and perfected alchemy and successfully performed human transmutation on himself.


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Why did Veldora have so much influence despite being imprisoned in the show Reincarnated as a Slime?
When Veldora disappeared, it was said to have a large impact on the surrounding lands. Why would it make such a large difference if he was already imprisoned and unable to use his skills?

By impact or influence, you mean....?

This is an excerpt from the first light novel, page 110. It only explains the Direwolves' perspective, but I think it can be applied to all the monsters

So why had they shied away from entering it [Forest of Jura] themselves?
Simple: Veldora the Storm Dragon. He was the one and only reason. Even
when he was within his prison, the waves of dreadful magical force
shook their very hearts. The creatures in the forest, they believed,
enjoyed the dragon’s divine guardianship—which was why they could
survive under those scorching waves. That was what they had to
believe. Otherwise, the truth would drive them mad.

In other words, fear and conditioning.
Also, I think Milim also mentioned in the Anime that the Demon Lords had an agreement with Veldora not to bother the Forest. It seemed to be respected even when Veldora was imprisoned and only once he was gone did they feel free to interfere with activity in the Jura Forest.

Typically in any sort of media that uses some sort of superpower, the characters have some sort of aura. This can be seen initially from Rimuru unknowingly exuding a powerful aura. I believe it was said in the LN that Veldora isn't able to control his aura that well, so regardless of what he's doing his aura is gonna be present.
Well first of all, assuming the seal initially blocks his aura or even suppresses it, the forest will have a large area that is no longer covered by Veldora's aura, meaning weaker life-forms can now habit the area that it once covered. The disappearance of something so great is bound to have a large effect. Later on, assuming aura/magic propagates through the seal/air, if Veldora stays imprisoned for so long, the area around him will be saturated with magic power from his aura. This can be seen when Rimuru is exploring the cave when he arrives and there is an abundance of rarer materials. Since the surrounding area is full of magic power from Veldora's aura, then if magic follows similar laws as matter does, it will keep spreading to try to keep an equilibrium of magic particles and eventually reach outside the cave, eventually covering another area and preventing weaker life-forms from living there again. The fact that he is still there causes a large influence, even if he is sealed up. When Rimuru devours him, his aura is no longer present, causing everyone that is capable of detecting that his aura isn't there anymore to know. Who wouldn't be nervous if one of the strongest life-forms in your world just disappeared all of a sudden?

Please include relevant sources/references.


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Are some unusual honorifics used in Shougeki no Soma actually used in regional dialects?
I noticed some unusual honorifics used in the Shokugeki no Soma anime series:

Yuki Yoshino tends to use "-cchi", as in "Erinacchi" or "Takumicchi"
Terunori Kuga tends to use "-chin", as in "Yukihira-chin"
Momo Akanegakubo tends to use "-nyan" or "-myan"

Akanegakubo's usage was described as a personal affectation to describe people that were cute (and also to look down on them a bit), but are the first two actual regional usages or due to a personal style/usage of the speaker?

They are informal honorifics. They tend to be casual and/or has an overall cute-factor.

I have done some research and added an answer. Hope it helps :)

-chi, -cchi, -chin are informal honorifics. They are a deliberate mispronunciations of "-chan", typically used by teenage boys/girls trying to be cute. Since most teenage girls want to be considered cute, they will tolerate this from close friends. They are used to build nicknames or use it to playfully tease/insult. Some also use this suffix to the informally name people that are cute and famous i.e. pop idols. Apart from food wars, I have seen this suffix being used by Yasuhiro Hagakure from Danganronpa franchise.
Coming to -nyan, it is pretty self-explanatory. It is used to express joy, satisfaction, cuteness. It is used mostly in anime. However, in this case dialect have somewhat of a relationship. Due to a specific dialect, a village in Japan end their sentence with "-nya/nyan". But in other places, you'll never see it being used apart from maid cafes (because it sounds cringy(?) [citation needed :/]).
Source:

What are the differences between Japanese honorifics chan, chi, chin and in? - Quora post

Japanese Names and Honorifics in Anime

https://gamefaqs.gamespot.com/boards/945498-shin-megami-tensei-persona-4/44399981

Those are not honorifics per say, they are suffix you can add to make nicknames, same as -chan. And they are certainly not limited to a prefecture, aka, they’re not from a dialect.

Please include relevant sources/references to support your answer.


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What happened to Magane at the end of the anime?
During the Re:Creators anime series, most of the creations from the other worlds ended up:

 dying, creating a new world (Altair & Setsuna), returning to their original fictional worlds, or staying in the creators' world (Meteora).

What happened with Magane?

 Since she didn't die or return to her original world, could one conclude that she also decided to stay in the world of the creators, albeit without any powers?

Magane...

 ...went to a foreign country and stays in the world of the creators as a normal human.

According to both Japanese Wikipedia and Pixiv Dictionary (Japanese):

 その後はフェスの顛末を見届けることなく会場を後にし、国外へ旅立つ。
 ほかの被造物とは異なりフェス後の描写はないが、広江礼威によれば彼女もメテオラと同様に能力を失うという。

 After that, she left the venue without seeing the end of the Elimination Chamber Festival and went on a journey to a foreign country.
 Unlike the other creations, there is no scene of what happens after the festival, but according to Rei Hiroe, she loses her powers just like Meteora.

and

 最終話(22話)で、メテオラの展開したゲートで元の物語世界へ戻った描写はない。
 これによって、数少ない被造物で現実世界に残ったメンバーとなった。

 ただし、広江礼威氏曰く「彼女はいずれメテオラと同じく能力を無くす。つまり、普通の人間になる」とのこと。

 In the last episode (episode 22), there is no scene of her returning to her original fictional world through the gate deployed by Meteora.
 This made her one of the few creations who remained in the real world.

 However, according to Rei Hiroe, "She eventually loses her abilities just like Meteora. In other words, she becomes a normal human being."

(Unfortunately, I couldn't find the original source for Rei Hiroe's statement)


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One Piece: Why didn't Spandam eat the Devil Fruit?
Note: I watched the anime, but did not read the manga.
In the Enies Lobby Arc, once the CP9 members return to Enies Lobby, Spandam gives a devil fruit to both Kaku and Kalifa.
But WHY?
Based on his childish and narcissistic attitude, I feel like he would want to eat them to be strong enough to do whatever he wanted. It seems out of character for him to give them both to someone, rather than use one of them himself. He loved having power over others, flaunting that power, and using it to cause pain to others. Was there an explanation I missed, or maybe the manga explained it somehow?

He didn't eat them because he didn't know what Devil Fruit they were other than that they were Zoan type, so he was scared to take the gamble. He wanted to eat a powerful fruit that he knows before eating; that was why he refused to eat one and gave them away.
If he eats one, then it is his one and only chance and only that Devil Fruit power can be used in his lifetime, even if he hates it. This fact scared him.

While accurate, you might consider adding a source to your answer to help solidify it.

I actually hadn't thought of that! He HAS read the encyclopedia, so it makes complete sense! Thank you for that!

If Spandam didn't know exactly which devil fruits he had on hand he wouldn't know if they were zoan, paramecia, logia type because it has never been established you can tell the type without knowing the exact fruit. Ergo, he couldn't have known he had 2 zoan fruit, if he thought he did then he was wrong because Kalifa's bubble bubble fruit is paramecia


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How come Neiru hasn't revived her sister yet?
In Wonder Egg Priority, when Neiru is first introduced, it's established that she started buying Wonder Eggs long before Ai, and often buys several at a time. Despite this significant head-start, Ai manages to revive Koito before Neiru is able to revive her sister. In fact, as of the end of Episode 12,

 Neiru is the only one of the four protagonists who hasn't revived their loved one yet.

How is this possible? Did Ai really overtake Neiru in terms of Wonder Eggs hatched? Or did Neiru have a higher target to begin with?

Following the release of the special, this hasn't been addressed, however it has been revealed that

 Neiru is actually an artificial human similar to Frill, and that her "sister" was actually her creator, Airu.

It's possible that Neiru therefore required more Wonder Eggs, by virtue of

 not being fully human.

This is speculation, but it's the only logical explanation I can come up with.
(Considering that the special raises more questions than it answers, my honest opinion is that the writers just didn't think it through, but I wasn't originally looking for an out-of-universe answer so I'm keeping that opinion as a footnote.)


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Do humans in Naruto who are descendants of Otsutsuki have any extra abilities over pure humans?
At first I thought only humans who are descendants of the  Ōtsutsuki clan and other humans were the ones who had the ability to use chakra, but according to other answers here and in other pages apparently not.
Do humans in Naruto who are descendants of Otsutsuki have any extra abilities over pure humans, or are they just the same than pure humans?

Of course, the descendends of Otsutsuki have extra abilities over normal humans/shinobis. They are in fact celestial beings that have travelled through dimensions seeking out worlds to plant Ten-Tail saplings into. So, basically they are "gods" rather than normal human beings. Some of the abilities are given in wiki (I am writing in points to reduce answer text):

the Ōtsutsuki ingest the Chakra Fruits from God Trees, giving them their godlike power and longevity

the Ōtsutsuki's had the ability to use what is known as "chakra" even before Hagoromo introduced it to humanity through the concept of ninshū.

during the Fourth Shinobi World War, those with the Ten-Tails' chakra and Kaguya herself are impervious to almost every shinobi skill (only by using Hagoromo's Six Paths chakra that she is ultimately defeated).

the Ōtsutsuki seem to remain unprecedentedly resilient even after they die (their soul wanders and manifest into other living beings)

Their dojutsu had special abilities: their Byakugan can unlock Tenseigan where normal Byakugan users can't do it. They possessed Rinnegan which had godly powers.

They are wielders of kekkei mora (advanced branch of jutsu unique to Kaguya Ōtsutsuki and her direct bloodline). Examples: All-Killing Ash Bones, Kaguya's Byakugan

but supposedly most ninjas are descendants of Otsutsuki . So why dont we see these abilities, is it because they are mixed blood or something?

@Pablo Kind of yes. Although they are descendants and not members of direct bloodline, the jutsu nature and teachings changed clan-wise and that's why we don't exactly see the same jutsu or jutsu of godlike power levels among shinobis. For e.g. point 5. Their jutsu power levels are toned down due to limited human chakra reservoir. One can increase their chakra level and be a "god" by being a jinchuriki of 10-tails and attain rinnegan just like Madara did.


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Is Kumoko really an unofficial name?
I'd heard Kumoko was an unofficial name for the protagonist of So I'm a Spider, so What?, so I didn't expect to actually encounter it in either the light novels but it's there. In Yen Press's translation of the first volume, it appears on pages 199 and 200, in Chapter 11: Still Clearing the Lower Stratum :

I mean, if I learned how to use these, I would basically be a magical girl... Well, a magical spider, anyway. Like "Kumoko the Spider Girl" or something.

Damn. It looks like I won't be able to call myself Magical Girl Kumoko for a while yet.

The name also appears in the ending songs of the anime. If this is an unofficial name, it seems it's been adopted. Do the original light novels really have something different in place of this text and never use the name? Or is the name merely considered unofficial because Kumoko's name appears as "nameless" when she Appraises herself?

From what I have been able to gather, the name is at best a late adaption of a fan name, given to the character to make referring to her easier. Which is why it's just the word for "spider" (Kumo), combined with a feminizing suffix (-ko).
The web novel's Character Introduction 2 (as I found it translated) states (minor name spoilers):

 Shiraori 白織  —  nameless 名前なし

 The protagonist, referred to as Shiro within the story and Kumoko outside the story, but almost never called by her real name. It couldn’t be helped that her name didn’t come out for such a long time. [...]

As you noticed, it has later started to appear in the light novel as well as the anime's ending.
And, of course, the name does seem official enough to be use by merch.

Sources:
So I'm a Spider, So What? Wiki - Kumoko - Trivia (heavy spoilers)
Kumo desu ga, Nani ka? web novel - Character Introduction 2 (heavy spoilers)
GoodSmile Company Nendroid Kumoko store page

I didn't link to the web novel translation, as it seemed kinda unofficial. So, I decided to treat it like a fansub; will refer, won't link.

I added the link to the original, untranslated web novel page instead.


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Did Itachi kill innocent people when in Akatsuki?
I was seeing some of the last episodes of Naruto and Itachi when in Akatsuki was ordered to perform missions to obtain money and power. In these missions the team usually had to kill people. Having Itachi killing innocent people to comply with his infiltration would put his character and the third Hokage who agreed with this, in a very different shade / position, possibly even putting in risk of potential war scale conflicts with other countries to the land of fire.
Did Itachi kill innocent people when in Akatsuki?

Unless there are details on these missions, it'd be hard to know if he killed innocent people.  Also, "innocent" in a world where war and inter-nation tension is common place is going to be largely subjective.

Did Itachi kill innocent people when in Akatsuki?

Canonically, there is little to no information regarding Itachi's missions while in the Akatsuki. So we cannot say for certain whether or not Itachi killed people during his missions; innocent or not.
If you consider some events that are shown within the anime, then yes - Itachi more than likely committed murder (innocent and non-innocent alike) while in the Akatsuki.
Let's look at some examples:
(Non-Cannon) While Itachi was in the ANBU, one of his specialties was assassination:

From his time in Anbu, Itachi became proficient in stealth, infiltration, and assassination.

Because of these skills, the Akatsuki utilized him (along with Jūzō) for these types of missions

Jūzō and Itachi worked very well with each other, being involved in at least three missions together, which involved assassination and sabotage.

(emphasis my own)

(Cannon) During the "Search for Tsunade" arc where Itachi is first introduced, he tortured Kakashi with Tsukuyomi, showing no hesitation. Following these events, Itachi orders Kisame to kill Asuma and Kurenai

Itachi instructs Kisame to capture Kakashi to find out what else he knows and kill the others..

If Kisame were to succeed, while not dying by the hand of Itachi, he gave the order to do so.

Having Itachi killing innocent people to comply with his infiltration would put his character and the third Hokage who agreed with this, in a very different shade / position, possibly even putting in risk of potential war scale conflicts with other countries to the land of fire.

In the public eyes, Itachi was viewed as a villain who defected from Konoha. Despite the reason behind the Uchiha's massacre, the true purpose was hidden to only a select few. If other countries within the Land of Fire were to declare war, it would be against the Akatsuki, not necessarily against Konoha.
Itachi had to do what he needed to do to maintain his covert mission. If he were to show any sign of remorse or hesitation on missions, the Akatsuki would have questioned his purpose for being there.


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Did Tobirama Senju actually invent the Kage Bushin no Jutsu?
In the anime Tobirama Senju tells Naruto he invented the Kage Bushin no Jutsu. However, in episode 465 we see Indra performing a cloning technique which looks exactly like it. And Indra was born before Tobirama. Also Asura tries it and fails to do it right. And they have an instructor who is next to Hagomoro. So it looks like either Hagomoro or the instructor invented that technique.
Is this technique supposed to have something different to Tobirama's, or did actually someone else invent the Kage Bushin no Jutsu ?

I think you are confusing the replacement jutsu with the clone jutsu

Canonically, Tobirama is the creator of the Shadow Clone Jutsu. This is confirmed in Naruto Chapter 644

According to the wiki (non-canon), Indra created the standard cloning jutsu; the one that Konoha academy students needed to perform in order to graduate:

Creating various techniques like the first Clone Technique and Body Replacement Technique, even Hagoromo was amazed at his eldest son's accomplishments, noting how revolutionary it was. Indra was a skilled crafter, having fashioned primitive kunai for battle and from which develop a basic form of shurikenjutsu
Naruto: Shippūden episode 465

Tobirama's clone jutsu was developed for espionage. A skilled shinobi could clone himself and go on intel missions. When the clone learns the information the clone jutsu could be dispelled and the host will get the information without risking death.


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Is Menma from Anohana half-Japanese half-Russian?
I saw a comment under a video which says the following:

A: I’m not sure but wasn’t menma half japanese half russian?

[...]
B: yes. the father is japanese and the mother is russian.
[...]
C: well, that's explain why she's the only one with unique hair in the anime
[...]
D: I’ve noticed in anime if you russian you have gray-ish hair. For example Viktor (Yuri on ice), Lev and Elisa (Haikyuu) and Menma

This is somewhat correct as Irene Honma (mother of Menma) is non-Japanese. But is she actually Russian? There is no mention of Irene being Russian in wiki apart from a note that says:

The name Irène means "peace" in Greek.

Question: Is there any information in source material(manga) regarding the actual ethnicity of Irene Honma? If so, is she Russian?

Related discussion: Is it known whether Kaori Miyazono is half-Japanese?

Yes, Menma is quarter-Russian.
Here is the quote from Wikipedia

本間 芽衣子（ほんま めいこ）/ めんま
.. ロシア人[24]の血を引く母イレーヌから受け継いだ銀髪碧眼と白い肌から、儚げな印象を与えるクォーター[25]の少女。

ロシア人＝Russian, クォーター=quarter.
The sources are:

[24] 『オトナアニメ』vol.21、洋泉社〈洋泉社MOOK〉、2011年8月8日、46-77頁。ISBN 978-4-86248-772-8。p50

and

[25] 小説版(novelized version)
『あの日見た花の名前を僕達はまだ知らない。(上)』、2011年7月25日発売1、ISBN 978-4-8401-3957-1
『あの日見た花の名前を僕達はまだ知らない。(下)』、2012年8月10日発売[2]、ISBN 978-4-8401-4689-0


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Was everyone alright after the camping incident?
At one point in the series, Hugo and his followers set out to kill Shun and everyone else during a camping trip. Given how determined and willing he was and how willing to kill everyone and cover it up, did everyone survive? The situation seemed dire, and it didn't seem entirely clear in the anime whether there were any casualties, since it went by so fast.

In the episode where this happens, after Filimøs/Ms. Oka resolves the situation, she says the following:

Shun, think you can go tend to Professor Oriza and Parton? ... It's okay, they're only slightly wounded.

It's easy to miss since he was introduced in this episode and really only appears in this episode, but Parton is the person Shun was talking to earlier, and the person who Hugo specifically attacked. So we can at least be assured that these two are alright. Filimøs's words generally give a positive outlook on the whole situation and seem to indicate that nobody died, but she doesn't really elaborate more than this. On the other hand, looking to the light novels spells things out more clearly.
In volume 2, page 188, in Chapter S5: Ruling Class, we get this:

Parton, Professor Oriza, and the others are all safe.It seems to have been a close call, but thanks to Ms. Oka's intervention, no one was seriously injured.All of Hugo's minions who'd attempted to attack people were captured.

So it seems there truly were no casualties. However, it's important to note that in the light novels, the earth dragon incident was actually part of the camping incident (and was actually brought there by one of Hugo's minions, though they lost control of it). In the anime, the earth dragon is instead affixed to the training incident where the students are practicing water magic. If I had to guess, this was done because they felt that Hugo needed further justification to hate Schlain/Shun enough to try and kill him (Yes, that's right: in the light novels, all Shun did to earn his wrath was douse his fire spell with some water magic. Furthermore, the books also go into detail that Shun tried to actively avoid dealing with Hugo, even back before they were reincarnated.). In the light novel's version of the earth dragon fight, Hugo's skills and stats have already been stripped by Filimøs, so of course he doesn't participate at all (though he is present), but other than that it isn't much different from the anime. And we get a wrap-up in the next chapter: From page 257, in Chapter S7: The Voice that Announces Ruination:

Wyrms and dragons...Hearing that, I can't help but remember that incident.Hugo's attempts to assassinate me, and the wyrm's attack on the school.Several years have passed since then.Although few were hurt in either assault, it was still a shock to the academy.

So, no mention of fatalities, and it seems that there weren't even that many injuries for either incident. As far as I can determine, it seems everyone was probably okay in both the anime and light novel continuities.


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Was Yuika Hasebe/Yuri a member of Mirei Shinohara's/Fei's gang that bullied Hiiro?
The reincarnations are all introduced very quickly, and while I did get an idea of who Hugo, Schlain, Oka, and Katia were in their past lives (albeit after some initial confusion for Katia, since in the English dub, Shun's other friend, the one who wasn't Katia, had a voice similar to Katia), Yuri Ullen's identity and past role didn't really get across, to the point where I didn't even know what her name was (I now know it was Yuika Hasebe). The only characters of note were the two who accompanied Fei in her bullying of Hiiro. Was either of them Yuika Hasebe?

In short, no, neither of Fei's friends were Yuika. Just going by wikipedia, Fei's friends were named Aiko Iijima and Kumiko Tonooka. Yuika Hasebe does appear in the flashback episode, though she doesn't do much to stand out. Here's what she looks like:

She looks a bit similar to Fei, but could not be mistaken for either of Fei's two friends:

In the anime, the reincarnations are introduced very quickly (unless I'm mistaken, all five of the characters I mentioned were introduced in the first episode, without much context, and it's up to the viewer to figure out they're reincarnations), and I feel this was done poorly and probably led to my confusion. The light novels have less of a problem with this, as the reincarnations are introduced more slowly, usually one-by-one, and their past lives are explained at least a bit.
Yuri's introduction in the light novels actually rules her out as an associate of Fei. From volume 2, Chapter S1: Academy:

The girl scolding Natsume was formerly Yuika Hasebe, who used to sit next to me.Unlike Natsume, Hasebe never made an impression of being particularly good or bad.Among the girls, the clique that stood out most was the one that included Fei, aka Mirei Shinohara. But Hasebe was closer with the relatively quiet girls, like Temarikawa and Furuta.Of course, that's just compared with Fei--she was still a pretty bright and active person herself--but Hasebe certainly never struck me as the type who secretly held the potential to be a saint or anything like that.

Back in volume 1, in the first Shun chapter, which actually takes place before everyone dies and is reincarnated, Hasebe is again described as "the girl with the desk next to mine". It seems she was intended to be a low-key character, one who even Shun doesn't take much notice of, whose basic identity to Shun is just the girl who sat next to him. I don't know anything about the original publication order of the chapters, but for the English light novels published by Yen Press, this is when Yuika Hasebe is first introduced, in the first volume, Chapter S1: The End of Normal Life:

"Ah, crap.""What's wrong?" Yuika Hasebe, the girl with the desk next to mine, responded to my grumbling."I forgot my pencil case""Oh, did you? Well, I suppose you can borrow these, then." Hasebe handed me a pencil and eraser."Thanks.""Mm-hmm. You owe me a piece of candy.""C'mon, you're charging me?" I groaned, but I smiled wryly and waved a hand in acknowledgement nonetheless.

It's a pretty normal conversation, but helps characterize Yuika Hasebe a little bit... as someone pretty normal. She's perhaps a little quiet, and definitely not a bully.


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How many chapters does the GATE Manga have?
I'm really enjoying the GATE Series, having watched the anime, as well as in the process of reading the manga. I'm also in the process of getting my hands on the novels of the series.
However, I've only made it up to Chapter 105, namely the chapter where Lelei tries to open another GATE for the JSDF. To me, that can't be the end of the manga. It's such an abrupt ending. After chapter 105, the trail goes cold.
Does the GATE Manga stop at Chapter 105? Or are there more chapters to be read?

The official web manga is on Alphapolis (Japanese). It is still ongoing and it's a monthly serialization. Chapter 105 was just released on 30 July, the next chapter is planned to be released on 30 August.

@AkiTanaka Thanks for the reply. Originally, I thought that the manga had died off some time ago, like the anime did. But now that I see that it's ongoing, that is beyond awesome. Thanks again!

According to MyAnimeList, the GATE manga is still ongoing. Chapter 105 is the most recent chapter and, judging from the age of its discussion thread, only came out recently.
So there are no more chapters to read yet, but there will be. You'll just have to be patient.

I don't get it. Why the downvote? BTW, thanks for the answer. Those links are real helpful.


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Why is Fei able to fly when she's an earth dragon?
Earth dragons are specifically mentioned to have no wings and be unable to fly, so how is Fei able to do so?
From when Araba first shows up:

Contrary to it's name, it looked more like a wolf.It had four feet that trod on the ground.A long tail.No wings.What I saw there was an imposingly majestic dragon.

This may not have been explained adequately in the anime (or perhaps I just have a knack for not paying attention). Firstly, note that Fei was an earth wyrm, not an earth dragon, and this series makes a distinction between these, as a dragon is a more evolved form (which Fei may reach someday). Now, for the answer.
Fei is able to fly because she evolves into a light wyrm. This is a special evolution that becomes available to her because she's the hero's familiar. From the light novels, volume 3, chapter S6: Hiding:

Fei had evolved, becoming a light wyrm.Apparently, this new evolution was suddenly available to her because I became the hero.Since we have a contract as master and familiar, my obtaining the Hero title had an indirect effect on FeiAccording to Hyrince, there have been similar cases in the past.When a hero has a monster familiar, that monster undergoes a a special evolution.This is how Fei apparently shifted gears from being an earth wyrm to a light wyrm.With her powers as an earth wyrm still intact.Earth wyrms are known for their exceptional defense power.So Fei gets to keep that defense, while gaining the ability to maneuver freely on both land and sky thanks to her new wings.Her newly acquired skills are Light Wyrm level 9, Light Magic level 1, and Flight level 1.

Though Shun doesn't notice at first, he also figures out Fei has a hard time flying straight since her Flight is still level 1, after he tells her to fly straight at an enemy:

I couldn't tell up until now, but it seems like Fei really isn't used to flying yet, since she hasn't trained the skill up at all.Apparently, flying straight is even more difficult than dodging the beam attacks.


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How did Kumoko know she could defeat a monster by making it run out of SP?
Unbeknownst to the viewers of the anime until the reveal at the end, the crux of Kumoko's strategy against Earth Dragon Araba was to make it run out of SP. How did she know this was something she could do?

Note there are two types of SP: Instantaneous (yellow) and Persistent (red). The red one is the important one here. Yellow is basically just how much one can run around before getting winded, while red is metabolic energy.
I went back to the scene where Kumoko's supposed to learn this, and I have to say the anime really messed up. It isn't as though it's preposterous for Kumoko to just figure this out by guessing, but the confirmation would be important if she's going to stake her life by using it for formulating a strategy in a life or death battle. She was supposed to learn this immediately after getting her first evil eye (the Cursed Evil Eye), when testing it out on a frog. From volume 2 page 169 in chapter 7: The Administrator's Shadow:

Hmm? Huh? The frog died?What the hell? I thought it still had some HP left...The rest of its HP drained in a flash. But why?Ahh, it ran out of red SP before it ran out of HP.So that's why. Guess when you run out of red SP, your HP starts decreasing really quickly.WHAAAT?!Yikes. I've had some pretty close calls after evolving and stuff, then.Good thing I had food at the time.Thanks to Overeating, my red SP hardly ever goes down, but I'll have to be extra careful from now on

In the anime in the same scene, you can see the frog's red SP is going to run out before its HP, but it cuts away before that happens, and just shows its health. Not only that, but Kumoko doesn't really comment on the SP here at all. So, what was supposed to be foreshadowing and informing the viewer of how SP works ends up not doing either of those things.
I'd say this was a blunder on the part of the anime as far as storytelling goes. As Brandon Sanderson says, "An author's ability to solve conflict with magic is directly proportional to how well the reader understands said magic." So when the viewers don't know about the SP thing beforehand, it diminishes how impressed they are when Kumoko uses it to win.


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What did Kumoko do to raise Taboo?
What exactly were all the things Kumoko did to raise her Taboo skill? I know she ate a sibling, but the other ones kind of went by fast. I'd like a list of them.

Taboo is an interesting skill, but some of the ways Kumoko levels it up are very uninteresting, as it levels up several times when she evolves. I will try to list how she got each level, but it isn't 100% clear in one case.
Level 1: Ate a sibling
She first gains the skill by eating one of her siblings. It comes with the Kin Eater title she acquires as a result. From the first light novel, chapter 2: A Rent-Free Home:

So I press on and finish eating my sibling's carcass. This is enough to fill my stomach, so I don't have to worry about starvation for a while.Whew. Thanks for the meal, I guess.<Condition satisfied. Acquired title [Kin Eater].><Acquired skills [Taboo LV 1][Heretic Magic LV 1] as a result of title [Kin Eater].> Uh, what?

Level 2: Evolved into a small taratect.
This happens on page 113 of the first light novel. Seemingly all Kumoko does is evolve into a small taratect. It doesn't seem like a particularly controversial choice. Skills often level up from proficiency bonuses while leveling up, so that could be what's going on here. Kumoko gets a skill proficiency evolutionary bonus while leveling up. If this is really the case, it doesn't seem particularly fair to automatically level up a negative skill like taboo, described as "Must never be raised." I'll note that on page 152, in chapter 9: Spider vs Bee, Kumoko says this about skills:

And with this latest level-up, three of my skills powered up too.I don't really know what any of them did, but if they leveled up, I must be earning proficiency for them somehow, so it's possible that I'm benefiting from them without even realizing it.

She's not talking about Taboo here, but if this is really true in general, Kumoko might be doing something to earn Taboo proficiency, and it just only happens to level up when she evolves. It's noteworthy though that she never gets a Taboo level up just from leveling up, unlike a lot of her other skills. There's really no way to know at this point, and I can really only go off of what the light novel says. Though perhaps relevant then, is when she gets the Merciless title on page 242, which comes with some more Heretic Magic and Heresy Resistance. She gets this title after slaughtering a large number of the monkey horde that comes after her. Her attitude at the moment is probably what gives her the title:

I sense a hint of fear in their cries, too, but what do I care about that?They're the ones who picked a fight with me, so they should've been prepared to die themselves.You're not gonna catch me losing any sleep over killing something that tried to kill me first.So I ignore their struggles and finish them off.

Heresy Magic is notably ominous sounding, and it also came with Taboo, so her proficiency for Taboo may have gone up here for one reason or another.
Level 3 and Level 4: Acquiring the skill Pride
Moving on to volume 2, on page 26 her Taboo level jumps up to level 4 after she acquires the skill pride for the bargain price of 100 skill points. Since she'd decided to live her life with pride, she may have gained an affinity for the skill which might explain the cost. And if pride is connected with Taboo, being prideful could again be giving her proficiency in Taboo behind the scenes (later on, with a maxed-out Appraise, Kumoko is finally able to see her current proficiency and how close it is to leveling up, but at this point, she can only see the current level). Taboo goes up immediately after she gets the skill Pride, even before she gets the title Ruler of Pride and the abyss magic that comes with it, so we can be reasonably sure that it's acquiring this skill that increased Taboo by two levels.
Level 5: Evolving into Zoa Ele
This happens on page 131 of book 2. Again, another evolution boost. It's noteworthy that Taboo doesn't go up for every evolution, as it didn't level up for her evolution to Poison Taratect. Zoe Ele is notable for being described as "A small-spider type monster that is feared as an ill omen", so it does seem more taboo than her past evolutions at least. For further comments, see Level 2.
Level 6 and Level 7: Acquiring the skill Perseverance
On page 139 of book 2, Kumoko acquires the skill Perseverance for 500 skill points. It lets her survive lethal attacks by sacrificing MP, and it has a lot of the same terminology in its description as Pride, and comes with a Ruler of Perseverance title, just like the Pride one.
Level 8: Acquiring the skill Wisdom from Administrator D
In volume 2, page 175 in chapter 7: The Administrator's Shadow, Kumoko is complaining to herself that Appraise didn't evolve into any new skill, but it seems someone is listening:

< Upper Administrator D has accepted the request><Now constructing skill [Wisdom].>< Construction Complete.><Condition satisfied. Acquired skill [Wisdom].><Skill Appraisal LV 10> has been integrated into [Wisdom].><Skill [Detection LV 10] has been integrated into [Wisdom].><Proficiency has reached the required level. Skill [Taboo LV 7 has become [Taboo LV 8].><Condition satisfied. Acquired title [Ruler of Wisdom].><Acquired skills [Height of Evil] [Celestial Power] as a result of title [Ruler of Wisdom].

It's not entirely clear why Taboo levels up. It could have something to do with the contact with Administrator D, but considering Wisdom and it's title seem similar to Pride and Perseverance, and considering Taboo levels up immediately after acquiring Wisdom, it probably is the skill that does it.
Level 9: Eating a Fire Dragon?
How Kumoko levels up Taboo to 9 is unclear in the light novels. It comes after having directly spoken with Administrator D following her defeat of the Fire Dragon in book 3. It does seem the death of a dragon is concerning, even to an administrator, as this is why Güliedistodiez tracks Kumoko down afterward (though is interrupted by Administrator D), as evidenced by this mysterious passage at the end of chapter 4: Dragon Slayer:

"A dragon's been destroyed? In the Great Elroe Labyrinth... Was this his doing? No, I don't think so. We have an unspoken truce going on. I don't think he would make a move like that. But what happened then? Activate Administrator's authority... What in the world? A ruler? Of three, no less? What does this mean? I've never heard of this skill 'Wisdom'. Does that mean this is the work of D? But why? What is this skill? ...I guess I'll have to investigate.

This is the passage where Kumoko levels up Taboo at the beginning of chapter 6: Middle Stratum Cleared:

I eat.And eat.And eat.It's bitter.So bitter.Bitter, just a little sweet, and sad.<Proficiency has reached the required level. Skill [Taboo LV 8] has become [Taboo LV 9].>

At this point, it isn't obvious whether or not what Kumoko's eating is dragon or not. It seems probable since she does check the skills she got from defeating it, so not much time could have passed, and also because she was descaling it when the Administrator arrived. Another possibility is thinking too much about the Administrators. This is obviously what's on her mind as she eats, and she's annoyed at them, thinking bad thoughts about deities including how she probably couldn't defeat Administrator D if she fought her, which seems like Taboo thoughts, so it's possible that's what raised Taboo, though it isn't really substantiated. Here's the passage:

Otherwise, that would mean this whole world exists as a sideshow for D's amusementI don't like the sound of that...Aside from that, I don't have enough information to do anything but speculate.So I have to conclude there's no point in racking my brains about "D," "administrators," and all that jazz any further.Honestly, it's not like I can do anything about it anyway.If beings that can control the skill system want to mess with me, I doubt I could put up much resistance.Trying to go up against an administrator would be like trying to stop a meteor from destroying the earth.That's not the kinda thing I can win against.Not just D, either. That dark guy who showed up in front of me earlier seemed way more powerful than me, too....Never mind a contest of strength. I wouldn't even be able to qualify for the game.

Later in the chapter, we do glean something that raised Taboo partway up to the level up though:

I also maxed out Heretic Magic.But you'll never guess what god-awful skill derived from that.Taboo.Luckily, it didn't level up, but I thought my heart was going to stop for a second.Actually, even though it didn't level up at that moment, it's hit level 9 at some point without my noticing.One more level and it'll be maxed out.This could be really bad.

Level 10: Evolving into Ede Saine
Like the Zoa Elle, this evolution does have a foreboding description: "a small spider-type monster feared as an omen of death.", but this may or may not explain Taboo going up. See Level 2 for further comments.
More Taboo
That's it for Kumoko's leveling up for Taboo, but here are a few more examples for ways to level up Taboo from some other characters.
Fei gains Taboo just for biting the earth wyrm that shows up to attack the students. Apparently, this is sufficient for gaining the Kin Eater title. It's probably that she ingested at least a small amount of the wyrm, but it is still a bit surprising. (Note, she didn't kill the earth wyrm, which was probably her parent; Shun did.)

Then I notice her new title.[Kin Eater]As its name suggests, it's a terrible title conferred on those who have eaten the flesh of a blood relative."It...can't be...""I don't see any other explanation, do you?"Fei must have bitten right through the wyrm's neck.If so, her grant of this title would make sense.In fact, that's the only way it could have happened.

From even further down the line, Shun's Mercy skill levels up Taboo rapidly.

 This skill allows Shun to resurrect the dead, and he uses it to save Katia after she blows herself up to free herself from Hugo's brainwashing, and after their execution Shun uses it to save Katia's parents and one of his elder brothers, even going so far as to put their heads back on after they were chopped off (he had to do this himself, as the skill wouldn't work if the heads were detached).

From page 166-167 of the third light novel, Shun's Taboo level jumps from level 5 to level 9 :

 Mercy. A forbidden skill that resurrects the dead.This is the skill Katia questioned me about, saying it was no ordinary magic. A skill that brings about miracles.That time, when Katia fired magic at herself to get rid of the brainwashing, she really was mortally wounded.Her HP dropped to 0, and she died.However, I used this skill right away and brought Katia back from the dead.Once I revive Leston, I do the same for Klevea and Katia's parents.In the process, my MP all but runs out, and my Taboo skill level goes up to 9.Mercy is probably the only skill in existence that can revive the dead, but it has serious conditions and disadvantages too.Firstly, it consumes a large amount of MP.The only reason I could revive four people was because my skills greatly increased when I became the hero.Also, if the body is too damaged, it can't be revived.This time, I was able to reattach the severed heads to their bodies, but if I tried it with only one or the other, it probably wouldn't have worked.Finally, it's only effective for a short period of time after a person's death.I haven't experimented with it, of course, so I don't know exactly how long, but my sense is that it's probably only a few minutes.

In volume 13, pages 105-106, covering the same event, Kumoko states that Shun's Taboo went up by four levels after resurrecting four people:

 I kept it to four people just in case Yamada ran out of MP, but maybe
 I should've thrown in a few more? From what I can tell, it doesn't
 look like his Taboo skill maxed out. It probably still went up four
 levels, though, so this whole exercise wasn't a waste.


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Did Minato give BEE his nickname?
Has been stated officially that it was Minato who gave BEE the title of shinobi killer? I seem to want to think so.

Yes, Minato did in fact call Bee a “Shinobi Killer” in Naruto Chapter 543. This occurred during Minato’s battle with B and the 4th Raikage.

The two from this encounter Minato would come to have much respect for B and his abilities. Minato even stated that B acted as a true shinobi killer, thus giving him the title of "Killer"


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Why didn't Asta and Secre just ask Marx to use Memory Magic instead of taking time studying devils?
I just finished watching Black Clover. I haven't checked the manga yet.

The title pretty  much summarizes my question. Why didn't Asta and Secre ask Marx to use Memory magic to prove that they are innocent, just like how Marx used it to know the truth from Catherine and George during the Royal Capital Assault arc?

It was because Marx was possessed by an elf, thus it won't change peoples minds. Also even if Asta was proven innocent, one way or another, someone has to be punished for the damages(the Magic Knights who were possessed) as said by Damnatio.


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Why did Mother and the Demon Lord want to kill Kumoko?
In the anime, all Kumoko does is refuse Mother's (Mother is here referring to the Queen Taratect who birthed Kumoko) order to come back to the Great Elroe Labyrinth after finally escaping it. Is there some deep reason that spiders aren't supposed to leave the labyrinth or something, important enough that it would spark a conflict between Kumoko and Mother? It just seems really odd they would both expend so much effort on one spider leaving.

This confusion stems from differences in the anime and light novels
The source of their conflict is actually pretty straightforward, and this is yet again another change in the anime that muddles the story a bit. In the anime, Mother tries to control Kumoko using her Kin Control skill, trying to get her to come back to the labyrinth, but Kumoko is immune by this point, likely due to Heresy Nullification. Immediately after refusing the order, a spider hit squad is sent after Kumoko, which she wipes out, and the conflict thus begins.
Mother attacking straightaway in the anime like that definitely contributed to making me think there was something mysterious going on, like some kind of spider covenant to stay in the labyrinth or something along that nature, but it's really not the case (not that I can really rule out something like that exists, having only read up to volume 4). But there's one big difference in the light novels that really makes this less of a mystery, and that is...
Kumoko instigated this fight
In the anime, it is after the events I just mentioned that Kumoko begins her psychic attack on Mother, but in the light novels, she begins it immediately after discovering her psychic connection to Mother, which happens even before her fight with the earth dragon Araba. This happens in volume 3, and she mentions that it was Mother's influence that made her get irritated at dragons, and that Mother only directly controls spiders that are powerful, and thus she discovers the connection when Mother tries to control her. Thus, while it cannot be said Kumoko wasn't provoked into the attack, she is the one who instigated this fight, and everything Mother does from this point on is just for self-preservation, as Kumoko's psychic assault definitely will kill her given enough time.
Demon Lord's Motivations
The Demon Lord Ariel's actions are directly tied to Mother's assault as well. Mother is one of her descendants (probably a direct one). It's a bit unclear what their relationship is exactly, but given spiders participate in cannibalism of their kin, most likely she just thinks of Mother as a very powerful ally, and wants to preserve her for this reason (it's said there are only a handful of Queen Taratects in the world, but they're all considered threats humans could never defeat).

 My assumption turned out to be wrong, as later volumes make it clear Ariel genuinely does care about her kin, even calling them her beloved children in some cases, and this is especially evident with regards to her puppet spiders and the lost Queen Taratect. She mourns the loss of her Queen Taratect, and even prays for her soul. Albeit, Ariel does still have the Kin Eater title.

Interestingly, Mother's Heresy Resistance is level 9 (see page 45 of volume 4), so it's unclear how well Ariel can actually control her, so it's possible Mother is more of a willing minion. Though there's actually a bit of evidence Ariel can control her, such as the passage below, so take my little theory there with a grain of salt. Whatever their relationship, Ariel tries to kill Kumoko to try and save Mother.
Further information on page 161 of volume 4:

Hang on--why would a demon lord come to kill me anyway?No, I guess I already know.The Demon Lord's species name was origin taratect.The zenith of all taratects, who even Mother would have to obey.Just like my Parallel Mind told me in that emergency contact, she's even stronger than Mother, the monster we thought was the strongest of all.She must have gotten some kind of distress signal from Mother and come to eliminate me.Well, I guess my victory was only a matter of time, so it makes sense that Mother would request backup.

Kumoko as instigator, again, with the Demon Lord
Volume 5 of the light novels actually shows the Demon Lord and Kumoko's first encounter from the perspective of the Demon Lord, on page 152. Ariel politely asks Kumoko to stop attacking the queen, but there's some miscommunication, since Kumoko can't understand the language of the world. In Kumoko's chapter, she tries to make gestures to indicate she can't understand, but Ariel interprets these as a refusal.

"Hi there. I'm your grandma."I jokingly introduced myself, approaching in a friendly manner."Sorry to ask this right off the bat, but could you stop attacking the queen now, please?"Hearing that, the creature titled its head, then waved a leg from side to side.It refused.It must have tilted its head to imply that it didn't know why I would ask such a thing."I see. I suppose I have no choice then."It's not that it didn't occur to me to try to bring the Ruler onto my team.However, I judged that it was too dangerous to leave this thing to its own devices.If it wasn't going to listen to me, I'd just have to crush it on the spot.

If there's still any doubt that there's some big reason spiders can't leave Elroe Labyrinth...
In volume 4, it's revealed that a Queen Taratect lives in the elven forest, outside the barrier (and is one of the main reasons they need the barrier). Another one was mentioned as having been used to destroy one of the human forts in the Demon Army's big offensive (that was probably in volume 2 or 3).
What about the anime?
Since things happen differntly in the anime, we probably have to say anime Mother's motivations are a bit different. A reasonable conclusion is that Kumoko disobeying her orders was irritating enough to her to send spiders to attack her. The psychic assault later definitely still comes into play to escalate their conflict and increase Mother's desperation to kill Kumoko. The Demon Lord's motivations probably can be said to be more or less the same.


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Are Sophia's parents vampires?
As far as Sophia being a vampire, the anime never really addressed why she was a vampire, and her parents never commented on it. Is one or more of her parents also a vampire?

The light novels answer this. In short, no, neither of Sophia's parents are vampires. And given that the light novels actually specify that very few people have Appraise, and that high-level Appraise Stones are rare, it's entirely possible they're completely ignorant that Sophia is one. Though Sophia's parents are nobles, so it's possible they have access to an Appraisal Stone. It isn't clear whether they know Sophia is a vampire or not (I'll update if necessary, as I'm still reading volume 5).
The reason Sophia is a vampire is that Vampire was the unique skill assigned to her at birth. That each of the reincarnations were assigned a unique skill is also something the anime didn't really explain (at least as far as I can remember). Kumoko's was the skill Skanda, which increases her speed. Note that these unique skills are not necessarily one-of-a-kind, only for this reincarnation, as plenty of monsters are shown to have the Skanda skill. Back to Vampire, this skill gives Sophia the Vampire title as well, which comes with some really good skills like Undying Body (Which was evidently mistranslated as Immortality in the English version in volume 4, and corrected in volume 5 when they realized it wasn't the same thing. To be sure, there's a difference, because it only allows her to survive any attack at 1 HP, once per day, whereas Immortality lets one survive any kind of physical attack, with Abyss Magic or other attacks on the soul being the only way to fully kill someone with it.). Another really interesting thing, is that since Sophia was assigned Vampire from birth, she also gets the Progenitor title, which negates all negative effects of being a vampire.
Textual Evidence
Getting back to the main question; this all comes from volume 5 of the light novels.
From page 4 in Chapter 1: The Spider and the Vampire:

Does that mean her parents are vampires or what?But according to Appraisal, the woman who's holding this baby is human.The lady's name is Seras Keren.Same last name as the baby bloodsucker.If you put two and two together, that means this lady is definitely the kid's mother.Her mother is human.

From page 6 in the same chapter:

So by the process of elimination, her reincarnation bonus skill is... Vampire?Hmm? Hmmmm?Which means that the reason this kid is a vampire is because that's what she got for being a reincarnation?The description for the Vampire title did say that it gets added to your species when you get the skill.

From page 30 in Chapter 2: The Town:

Actually, aren't they nobles?From what I saw earlier and all, I'm guessing the baby bloodsucker's father is in charge of the town.His name is John Keren.Race: Human. Human. I say it twice because it's important!Good for you, Baby Drac! You're a vampire who was born to human parents via some freak mutation!...I don't know how vampires are treated in this world, but if an important noble suddenly has a vampire baby, that smells like it'll be trouble in the future.Well, they'll have to deal with that themselves.


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What does the n%I=W skill do?
All of the reincarnations have the n%I=W skill. What does it do? I don't think it was actually explained in the anime, and it was pretty much kept as a mystery.

This was explained by Administrator D in the fourth volume. It seems this skill is what allows them to retain their memories of their past life. It also seems to be why their stat growth rate is so high, however, this may only be an indirect consequence of the skill and not an actual effect (I'll emphasize the point in the passage that makes me think this).
From D's speech on page 103 in Chapter 5: Evolution (emphasis mine):

Left to their own devices, their souls would have disintegrated. That's why I took matters into my own hands and gave them the 'n% I = W' skill. With this skill, they would be able to keep the strength, memories, and such of their souls. Then I gave each of them a free skill based on their aptitude and generously ensured they would be reborn into species with a similar wavelength to their original souls. If you ask me, that was the least I could do.

The reincarnations being able to keep the strength of their souls may be why their stats grow faster than regular people, and thus it isn't actually that this skill directly makes them have better growth rates, though it is an indirect consequence.
Further evidence for this theory on page 144 from Kumoko:

And it would be no exaggeration to say that the power of stats, skills, and so on in this world is actually the power of the soul.

It's possible there may be more to this skill to be revealed later, but this is a pretty good explanation for now, and it isn't even much of a spoiler.