The issue is lack of epilogues and supposedly Larian is working on them so hopefully it's a matter of time as it would fix most of the issues you mentioned. As for Karlach, I'm pretty sure she can be convinced to go back to Avernus so maybe you messed up something with her questline
Playing as Astarion you probably can't convince her because he's too busy burning in the sun himself.
And his garbage ending has its own thread too.
