Has anyone done any testing with scenes like where Lae'zel runs away to meet with the gith patrol ..... and see how that functions if she's the only living party member?

Looking at how the game handles stuff like that, or the post-grove battle party at the camp or the asterion bite scene might be good to see how the game handles a dead main character.

I kinda suspect you would get a game over since you would have no living party members in the first instance and that the game just grabs a random party member to do the speaking in the case of the latter. Some scenes probably just don't trigger at all, like romance scenes.

Anyways, The game is set up like DOS and DOS2 so that everyone is a sorta a main character, both from a story perspective and to a lesser degree- a gameplay one. This presents a lot of hurdles IMO that Larian has to work with. In the original BG, if you did multiplayer IIRC the other player had no say in dialogues and pretty much just followed the 'primary' player's lead, so the story was still oriented around a singular main character. Not the case here. BG III seems written such that barring special 'character moments' events and dialogues are more oriented around addressing the party and the tag system of whomever is the 'speaker'.

I kinda suspect that aside from the inability to initiate conversations with other party members, the game will hardly notice the MCs death. The perspective of certain party member scenes might change if only one party member is left alive. You can already initiate conversations/cutscenes with npcs with your non-MC party members.

Anyways, I won't add any more except to say that I'm not a fan of this system as it is in BG III. I feel like it is going to have similar shortcomings compared to DOS2 in that while it may have a more robust tag system, the game being unfocused in regards to the main character, particularly if you roll a custom character. Too much has to be surrendered to make the Origin system work, to make the multiplayer work the way they intend.

I do have some confidence that Larian will work in a 'Tav-exclusive' storyline at some point. Hopefully, maybe. Since IIRC they did mention they didn't want to have that particular same failing as DOS2 did with their custom MCs.