Instead of full-on restarting, why not retrain to Devotion for Astarion's quest and then flip back to Ancients after?
I mean, I guess, but I doubt they'd ever get that far without being in the party. Shadowheart's chances of bringing down Viconia without allies are essentially nil, tadpoled Ulder Ravengard finds his son repulsive etc.
True for Jaheira but then she'd just be dedicating her time to trying to save Minsc. Or if they leave together, Minsc would run into the same tadpole problem and she'd have her hands full with that, assuming she survived.
I think the only way to get Halsin to leave in disgust is to not cure the Shadow Curse, which he can't do on his own so he'd be as good as dead. I'm not sure if there's a breaking point for him in Act 3, even if you embrace Bhaal.