I agree, though I think the solution isn't a simple one, nor implementable at this stage of development.
Probably ...
I mean, i honestly thought that if you turn that twitch integration on, whatever gets most votes at the end of voting is acutally selected ... but im no streamer myself, so i cant really test it ... and all videos i found nobody ever bothered to let it go to the end, they simply allways waited few seconds and then they picked whatever had most votes themselves.
So, this was most likely my misstake ... bcs if twitch integration works exactly this way, so people only tells you what they want, but choice is ultimately allways up to you ...
Then indeed we have no usefull tool for multiplayer, except telling curently talking character what we want. :-/
It kinda sucks, but i gues its better than nothing.

A) embrace party dynamic (like let's say Wasteland2) and allow us to freely switch between available party members
And i dont think it would be too hard to implement to be honest. :-/
I mean, just imagine:
Your Tav is talking with Kagha ... option to persuate her appears ... but your Tav have low charisma, so you want to use Wyll.
> You switch to Wyll (this option is allready in game)
> As you control Wyll, you simply click on either Tav or Kagha ... therefore you join ongoing conversation (bcs Wyll *IS* there the whole time) ... wich gives you exactly the same conversation screen ... and you pick (and most importantly talk) as Wyll.
> Later, if you want to switch back to Tav, you simply repeat the process.
I mean, what problem it can cause?
Maybe tags ... if your Tav is a Druid or Tiefling ... there are tagged options that Wyll shouldnt have ...
But since the game (as far as i know) should be prepared for case when Wyll is doing all the talking, it should be prepared for this aswell ...
The only question is, if engine is capable to interupt ongoing conversation, and replace it with another at same place, or other way around ... or if there is some kind of engine (or, lets be honest, Larian capability) limitation, that only allows conversation to flow in single way. O_o
B) just acknowladge different party members if appropriate - so still have "face" of the conversation but allow players to use appropriate companions for relevant dialogue choices with specific tags, and pick character's with best chances to pass a skill check.
That would be my prefference ...
I mean, my Warlock fails most of Intelligence checks, since its my dump stat ...
Would be really nice if Gale would roll the same checks aswell, and offered explanation of things instead of Narator. :3
That, and ofc option to use both:
[Persuation] Please, let the child go.
and
[Wyll][Persuation] Please, let the child go.
But that is basicaly the same scenario as abowe, minus need to pick another character and join conversation ... it would be much more fluid tho.
But that all was said countless times.

If Larian wants to do something like it ... they would ...
And if not, then i gues we have to deal with it somehow. :-/
But i think we all can agree on it holds incredibly huge potential.