If Larian goes with the party exclusion again, then yeah, I agree it's very likely their various reasons for betraying us will be the leverage used at the end of the act to justify that. I'm still saying that it's weak and uninspired story-telling, and mechanically dissatisfying to boot, and I hope they don't do that again.
Agreed - I didn't like that aspect of DOS2 at all. I didn't enjoy fighting my former / would be companions and hope they don't repeat that.
I started playing DOS2 with some friends a few years ago, but unfortunately, we couldn't finish it due to scheduling issues (this seems to be a recurrent theme for us whenever we make an attempt to play
any kind of roleplaying game), so I never saw how this played out.
But I wouldn't like to fight former / would be companions either, and I would find it to be a bit uninspired/forced, too, especially if the companions would automatically antagonize our character if they are not in the party.
I could imagine only
very few situations where it would make sense to me that our character and a former companion actively fight each other (e.g. if our Tav hands Astarion over to Gandrel, I think it would make perfect sense for Astarion to fight us if we were to meet again).