I agree with you, to a point, at the end of the day not only are these characters going to be in your party, they're also potentially going to be the player character. That means they need to believeably be able to react to situations as the player would like. I don't think an Astarion PC will be barred from saving Arabella from execution, but that will be part of a character arc, instead of part of the character. And so will hopefully our actions influence the behavior and, for lack of a better term, alignment of the other characters.
We could also get ourselves into a scenario where we haven't done enough to inculcate better virtues into the others that they abandon us in pursuit of their own agenda. I thought that might be what split the party up after the first act, but it seems that's been dropped by Larian.
i think every origin character has a deadlock, or more than one deadock. gale's deadlock is mystra. atrarion's deadlock is a vampire's corrupt. wyll's deadlock is mizora. lae'zel's deadlock is her queen. shadow heart's deadlock is shar. karlach's deadlock is zariel. the dark urge is deadlock is his/her dark urge.
with the deadlocks, the origin characters' altitude will be limited.
i suspect the true path only can be triggered by the non-origin character -- tav.
so the origin characters are for replaying to see bg3 again via legend characters' view. the origin characters are legends, but they can not be a possible savior. the possible savior only the player character tav can be, if tav chooses the ture path.
