I might add two inconsistencies to the list right off the top of my head:
1.
When Karlach is informed she will die by Dammon at Last Light Inn, some companions have comments, but notably Astarion says nothing about it. Moreover, the comments that exist about her dying are all the companions accepting it and just �enjoy your time left�. I feel like at least someone would be more distraught, feels quite cruel. On the same note: if you play Karlach Origin, this scene and reactions remain the same. You also do not get to touch anyone of your choosing and the entire thing is swept under the rug. Horrible reactivity.
FIX: Rewrite the reactivity and add for those characters who don�t have any. For a play as Karlach origin, the entire scene should be reviewed and Dammon should not be a creep and �steal� the moment.
2.
In my endgame, Karlach died during the battle with the elder brain, but during the ending scenes and epilogue she was there - burning to death and all. I also faced the same issue as OP: I wanted to choose going with Karlach to hell, but that broke my ending/epilogue with the origin I was romancing (Astarion). When choosing to go with Karlach without her being your romance partner, there�s no �Avernus� epilogue scene - just fade to back and credits.
FIX: Coding fixes for the non-dead issue. Allow player to ask romanced companions to come with to aid Karlach if they choose to go to hell with her - the timing of Astarion running away too should be reviewed as it would break the scene in that case. Then, allow non romance partners of Karlach to choose to go with their friend and get the epilogue nonetheless.
Tbh I wrote an entire thread on companion interaction because it was at such a ridiculous level at times, you can find it here:
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=896168#Post896168Dammon died for me at Last Light Inn so I didn't know this particular scene, I tried to not save scum, at least, outside all the bugs. I will add it.
