How to say "where else", "who else", or "what else" in Chinese?
Next, from the sentences above, it seems like when you place 还/别的/其他 or any other words that mean "else" and any question words like 什么，谁，and 哪里 together, you can already form "question word (like where/who/what) + else". I don't know if my assumption is correct though. Please kindly tell me if my assumption is incorrect.


Yes, a 'question phrase' can be formed by the way you have discussed.

For example:

你还要买什么？ - What else do you want?

你还要去哪里？ - Where else do you want to go?

还有谁要来？ - Who else still wants to come?



谁还 - Who still (want to come)

哪里还 - Where still (want to go)

什么还 - What still (want to buy)



In the 8th sentence, '女士，还有其他什么事吗?', it is actually okay to remove the '其他' as it is unnecessary as you said. (double else)

This forms: 女士，还有什么事吗?



"有" literally means "have"

"在" literally means "there"

There is not much meaning and explanation to why these two characters are sometimes used and sometimes not. I don't feel like I can fully explain to why this is so as if I did I have a possibility to give wrong advice. If you give me some comparison examples I would be happy to help you explain the differences to why one uses these characters and why one don't.