Astarion: you can give him to the Gur and there is a vampire ambush in Act1 B that related to his "origin moments". Will probably also betray you at Moonrise Tower as he wants to control the tadpole, not get ride of it.
I once said that it is unlikely we can keep a full party before Act 1 even ends.
Well, as long as half the party doesn't randomly perish DOS2 style I'm fine with them going their own way... now on to that spoiler piece.
I honestly hope that there WILL be a way to talk Astarion out of leaving the party cause I wanna remove the damn tadpole, not keep it. I wonder if the power of steamy loud sex would change his mind lol. Although I highly doubt it can be removed at MT. I've said it many times before, and I'll say it again: my prediction is it's gonna stay with us at least until act 3. Unless dataminers can prove me wrong, go ahead.