As for the part about locking out some characters, I think the reason why it doesn't really make sense for Astarion and Lae'zel to leave the party if you help the Grove is because I don't see them as particularly caring. Both of them would just maybe shrug their shoulders and accept it as just some petty thing and maybe an annoyance, but I don't see them being particularly ticked about it. Joining the goblins and murdering everybody in the Grove is definitely something that would go against the grain of the other two, so I can understand why they would leave.
I guess someone could make a case that by helping the Grove you are wasting precious time that could be being used to find a cure or to learn how to control the tadpoles. So the two of them might leave simply because they feel that you are not taking your condition serious enough and you are just wasting time helping a bunch of people. However, if you helped Lae'zel confront the patrol, I could see her being a bit more tolerant especially if she thought that maybe by helping the Grove you might potentially get cured by Halsin. She may not put a lot of stock in him that he can cure them, but she might be a bit more open if she felt betrayed by her own people.
And Astarion would probably be the same way. If by helping the Grove he might learn how to control the tadpoles, he might be a bit less against it.
I guess I don't mind if they lock out certain characters as long as it makes sense. If at some point you have to make a choice, and by making that choice it makes sense that you alienate a couple of origin characters, I'd be okay with that. Like I said, I want more options for more companions, but if there is a story element that really makes sense, then that would be fine.
So, for example, if at some point you have to make a choice between joining the absolute or rejecting the absolute, and two characters are all four joining the absolute well two characters are absolutely against it, with one character being on the fence, which might be exactly what they're planning since you are bound to meet the absolute at moonrise towers, I guess that would make sense. Which path are you going to take? Are you going to be evil and join the absolute, or are you going to be good and resist? Two characters want to be evil and two characters want to be good. I still think that's a bit too black and white for all these characters, but I would get it if they did it that way. And again, that's really no different from Pathfinder right at the very beginning where you are choosing companion characters based on some sort of story decisions. I might not really like it that way, but as long as they explain it and it makes sense from a character development standpoint, I won't be too mad about it.
I thought about a scenario where it would make sense for Astarion, for example, to insist upon joining the absolute. If you absolute was to promise him total freedom from his master, I could see him saying goodbye to the party if they don't join the absolute. It would not make sense for him to stay in the party at that point.
