What can I do to ensure that my friends' first game of Arkham Horror with me is as awesome as possible?
I've done something similar for some of the games of AH that I've been in. Not to the extent of adding theme/flavor into the cards and allowing playing input during encounters though; I only read encounters and mythos cards for them.

To start off, I'd be wary about pulling out all the stops to create an involved role-playing AH experience for a group's first game. Even if there is someone present that knows all the rules, there will still be rule misunderstandings, one-off nuances that need consideration, and re-explanations requested, which will break the thematic stride that you're striving to establish. But more importantly, (and this partially depends on how large the group is) games can take a long time. My group has played Arkham at least a hundred times (sounds a bit ridiculous to me, but we've been playing over 4 years), and we still have games that run into the 4+ hour mark. This can be a bit long for new players that don't know what they're getting into, and inundating the game with extra theme (along with this being people's first game) will only make the play time longer. My advice would be to get your friends to enjoy the game and understand gameplay better before going for a fully immersive experience.

That all said, here's what I did when I ran as the 'Keeper'. The first thing that ruins Arkham for new players (besides game length) is how vicious the game has the potential to become. I've found that a nice way to control the difficulty of the game is by drawing two or three mythos cards rather than one every turn, and the pick the one you'll put into play. This helps prevent the same gate from monster surging 4 times in a row and flooding the streets with things the players can't handle, as well as allowing you to give the game a quick boost by throwing the players a curveball if they're doing too well (drawing someone through a gate, destroying some clues on the board, moving some monsters directly in the path someone was planning on taking, etc.) In the future, it could also allow you a little leeway in picking headlines to enact if you're looking for a thematic progression. You can do the same for Arkham or OW encounters as well. For the encounters, one way of working towards the RP experience with going to the full extent that you described (allowing the players to invent their own responses) is to not tell the players what happens should they pass/fail until after they've rolled/spent clue tokens. In my group however, I've noticed that this can irritate some players. After they've played a number of games where they know the entirety of the encounter before making the check, they aren't keen to give up that slight advantage for some extra perceived theme. Anyways...
You could even do the same with monsters; pick specific monsters that you want to throw at people. McGlen have a particularly close match against a Shuggoth earlier? Now there's one back for round 2! It's also a good way to make sure the board isn't flooded with particularly weak/strong things and ensure that the Dhole gets his requisite board time.

The scenarios that were released for the Arkham League are good for a continuous campaign of sorts; the 'stories' from one game to the next are loosely tied together, but that's about it for theme. There are some extra gameplay conditions that the scenarios will throw in there, but that may just confuse new players if they expect those rules to be in subsequent games.

In short, make sure your friends enjoy the game for what it is before adding things to it (helping them to realize this if necessary of course...). All of the flavor and theme in the world won't make someone enjoy this game if they just don't like the mechanics in the first place.

Good luck!