To be honest, I'm curious about Gale. Do I understand correctly that romantic Gale wants to take the crown anyway, and friendly Gale wants/needs to depending on some factors? In my case, friendly Gale himself said he would give up the crown, and there was no option to convince him or suggest he take the crown for himself. He decided his quest on his own without any influence from my side. I wonder, of course, what that depends on.
I can answer for the friendly Gale, as my romantic interest has always been that damn vampire. So here goes. I can't remember at what point, but I think it's in Act 3. Gale tells the player character that he wants the crown. And you can agree with him, or you can start talking him out of it and get his disapproval. He is then summoned to his place by Mistra. There too, either tell her the plans, or just listen to her, or refuse her. And lastly, the last. If all along the way you supported Gale and his crown idea, then at the wharf, when you pick the line that you take it and become a god (there's also, like, you can challenge Mystra, but that's a bad ending). Gale agrees and in the epilogue we get Gale's God. If, on the whole route you talked him out of it, then on the dock, even if you say "to hell with it, take the crown", he will say, and then what? No, I'll give it to Mistra. And he'll give up the crown himself. So with a friendly Gale, you can see the influence of the player. You either indulge him or you don't, and he ends up making his own decision.
I had it like this: I want the crown - yes, listen to Mistra, but don't talk about plans, take the crown - God's bottom line is Gale I want the crown - no, listen to Mistra, but don't talk about plans, take the crown or give the crown to Mistra - Gail gives up the crown. I may have missed some more dialog, but either way, agreeing is god, but, if you talk him out of it, it's just Gail.
Unfortunately, or maybe fortunately, this is not the case with Astarion. You can indulge him all the way through about the ritual and then just talk him out of it. Or vice versa, talk him out of it all the way and then help him. There is no player influence on Astarion.
