I don't know if this was mentioned yet, but I think there is a fairly decent solution to the dialogue issue that Swen said that you have looked into but decided against due to complexity (choosing the speaking character).
I also think that most people would be happy with this change even if it isn't the one they exactly wanted.
If you were able to change the speaking member only at the beginning of the conversation through a simple "Narrator: you step in front of [Speaking Player]" - the narrator could then speak for the other character to have them reset to the new speaker: "Shocked, they turn to you" - cue the line meant for the new player character speaker. This from an outsider's view ought to solve a lot of the complexity problems by avoiding them in the first place, and giving a narrative reason for why an npc might change their tone or how they might react. MInimal (hopefully) extra work on any systems, becuase it just resets to the beginning of the conversation.
I think the big thing that people are most upset by is not being able to have autonomy on how they want to handle a conversation, so they feel they have to reload a save to have that autonomy.
I felt the need to offer this suggestion because I see the community really wanting the autonomy and Larian (or at least Swen) seeming to think that the players want to switch at any point in the conversation, which is certainly too complex to solve with WAY too many permutations.
TL;DR: All that anyone really seems to want is a way to reset the conversation to a new speaker at the very first moment, rather than have to redo an entire fight. I can't imagine, even with all the state and tag systems that this is TOO complex to solve in that way. But - hey - I don't work this project so who knows.