I answered your questions ...
Will you do the same?
No, you ignored the actual questions.
How would a devotion paladin Astarion work considering it was his job to lie? He also would not be able to lie in any of the cutscenes and dialogues (same would apply to devotion paladin Gale with his tales or devotion Shadowheart?)
For respeccing to work no origin companion would ever be allowed to use or reference any class ability in a cutscene or dialogue, in a game. No bear scene from Halsin because he might nit be a druid at that point, no display of magic from Gale at any point ect.
And the question about Shadowheart was not about respeccing her into another class (but that too can be problematic, see the devotion paladin) but resoeccing her into a cleric of a different deity.
Hard to worship shar if you are a cleric of Tyr.
For all this to even remotely work the story of those characters would need to be grinded down to the most generic, cookie cutter thing imaginable. Or Larian has to do extra work to make cutscenes for every eventuality. And both are simply not worth it.
