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.
