I can't be bothered to look for exact quotes, so take it all with a grain of salt, but if my memory serves me well:
1) Yes, like D:OS2, BG3 is an act based games. So once you reach the end of act 1 in BG3 (assumingly Moonrise tower) you will move on to a new area (Baldur's Gate?)
2) There might be a hard or soft level cap per act. When asked about level 5 in EA during one of the Panels from Hell Sven said (paraphrasing): "It is still not decided if level 5 will be in Act1 or not. Keep asking and it may happen". The question was about Early Access though, so it is not clear if he meant adding it into early access (which is act1 content) or if its apperance in Early Access depends on Larian allowing players to reach level 5 in act 1 in 1.0 release. I didn't pay much attention to level progression in D:OS2 but I think someone mentioned that game had soft caps per act. Same could happen in BG3 - it makes sense with rigid act structure, that way you don't have to do everything to progress, but completionist won't be overleveled for all future content.
3) In very early pre-Early Access interview Larian representative said that we will be able to pick and choose companions during act 1, but have to commit after that: that could suggest potentially lossing "spare" companions sometime around completion of act1, like we did in D:OS2.
