Sounds like a splendid game! We're eagerly looking forward to seeing it!
1) I've been toying around with a script action that would allow changing the player control to another bot. But it's still a rough prototype, and I'm not sure if I can make it work well enough to be reliable. There are many things to consider, like which inventory the player controls, and what the old player character is supposed to be doing while we're controlling the other character. If I were you, I'd leave it optional, and use it if the functionality gets into the game.
2) Lip sync is always a tough one... Just kidding, you can simply make it play a sound file, we have full ogg streaming support, and it's easy to add support for the player being able to skip voiceovers, or maybe make the dialogues automatically continue when the sound has been played. We're always eager to help you in any small way we can, don't forget that!
If you're going to make it an adventure, I realize there may be many RPG things you might want to change with the interface, such as leaving out the stats/talents/skills/minimap and so on. I'm open to suggestions, leaving stuff out is always easier than putting new stuff in!