I’d hate to say it but I’m just not sure this dev has any interest in plausibility. Little things like downplaying the other characters uniqueness, establishing why the PC is in charge just didn’t make it into the priority list.
I would disagree with that one. I think Larian has an interest, they just deemed their game to be good enough to present it to a greater audience. Don't forget thats it's supposed to be in EA for at least another year and the content so far is, all things considered, not in a worse state than many games coming out in their 1.0 version. We are complaining on a pretty high level because we want this pretty good game to enter the level of "masterpiece".
Just the fact that we are complaining that the companions do not act as we think they would proves that Larian managed to shape their personalities pretty well. It would be really bad if there were many situations where we couldn't guess how a certain companion would react. So the first part is done in my opinion. Now Larian just needs to let these companions act according to their personalities and find a way to do so.
Reworking a few key dialogs that are used to initiate, finish, or decide quests or dialogs that branch between options how to solve a quest are a perfect way to create memorable scenes. And it wouldn't use as much ressources as rewriting every small little meaningless dialog most players won't even see (they can still do that if they want and have the time)