I've just updated my GOG game version to 4.1.1.3669438. And there IS immediate changes, I've got new dialogue after the battle with Gortesh:
I can now try to convince Karlach that it would be better for her to go to Avernus, because it wouldn't be for long, and we should try to search for the ways to help her at all costs. She provides some counter-arguments and is not convinced in the end, however. These are already the steps in the right direction.