Edit: Also a good way for Karlach to address any unfinished business in Avernus, that is her style after all. Some check-list items crossed off.
I was with you up until this part.
I think it's a very jarring tonal shift to have her lusting for life and deathly afraid of returning to Avernus most of the run until the last act where she does a complete 180 and becomes a doomer who's suddenly fine with their fate and fine with going back there.
The whiplash that this tonal shift creates kills much of the rich emotional story beats that you referred to in your post. They need to either do away with that shift or lay the seeds for it in earlier acts depending on your choices.
The prime reason people love Karlach so much is her energy and her personality. She's a gentle giant who doesn't take any shit. And she looks damn hot to boot. That's what made most people fall in love with her. The infernal engine story takes a back seat to that.
What Larian did with Patch 2 was to suck out her vivacious and upbeat attitude and replace it with a somber, downer attitude that doesn't fit her at all.
I agree that keeping the drama is important, but it can't be at the expense of the character.
I think this is where some of the dialogue and exploration of Karlach's character needs to reflect the possibility of returning to Avernus and making it a choice she can live with. Literally add in dialogue options to steer her from doom talk to glass half full and you're alive like Dammon said, the world would be a better place with you in it, even in Avernus. Making a difference for the better.
Make something good out of it, purpose behind validating the option. The reality of returning to Avernus should've shifted throughout the game, she made powerful friends and allies. Enough so, that a couple of those friends are willing to return to Avernus with her, removing the one thing she hated most about the place, loneliness. This could be another dynamic to the evolution of her character based on player choices and approval rating with Karlach, leading into an alternate ending twist.
Exactly. If we're to have her return to her most hated place, we have to do some more narrative heavy lifting than the few nudges here and there that currently exist.
