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.
By "true path" I can only assume you mean good alignment ending or best ending in terms of saving all or as many as possible of the innocent and good people while getting rid of all the bad villains? But that's a false tag I think to call something a "true path" because it implies some canonicity or higher importance for it or implying the other paths aren't equally valid. But this is the end of this trilogy and this story...there will not be a Baldur's Gate 4 and if another BG game is made it will be a completely unrelated story with new villains, story arc, etc. So there will never likely be anything like Larian declaring any one path, ending, or set of events canon for the sake of continuity or anything of the sort.
But to go past that I also believe you are wrong about origin characters not being able to get the best good ending, whatever that might be. Sure origin characters will face different challenges but they will be able to move past them in either good or evil directions. Even the Dark Urge you can play trying to completely resist and that will lead to its own story and narrative though overcoming the dark urge is probably the most difficult because the more you resist you have to start making saving throws so if you fail that can cause issues but if you make them I'm sure that will lead to its own interesting path and story as well.
