What could be done to preserve dialogue flow is having the button for initiating combat always be there, but disabling it during the parts of dialogues where it wouldn't make much sense to have access to it? Like, in case of Astarion's first encounter, have it off while your character is on the ground with a dagger at their throat, but enable it again once they are free.

That would require combing through every single conversation to account for such situations though, and I doubt it's something anyone would have any patience for, especially given that there's a lot more of the game still out there, and there's already a lot of dialogues. Then again, they did re-record the narrator's lines, so... who knows?