do you think there will be a Desati (or whatever you call it, where you go in the beginning of KH1 and where you battle the Twilight Thorn in KH2) in bbs? If so, for Aqua, Terra, or Ven?
most likely ven, and least likely aqua. I think that there will be a destati in BBS because BBS should explain that whole thing. The voice, the platforms, the endless abyss that surrounds them, it seems like there's something about destati that we don't know yet. It's just too big to be what we've seen of it so far.
MAYBE the voice is Terra? (It's like you see a microphone at the place where you battle Terra in KH2FM+)
Yes. For all 3 of them. The game needs for you to test out your moves and abilities before you start your game.