Just made an account to post here. There's nothing I could add that hasn't been said already about how Karlach deserves a happy ending, how much the character has resonated with me, how much the VA absolutely killed the role, improved my playthrough...everything everyone's already said.
What I will add is an extremely important consequence of Karlach's poor conclusion, at least for me: I have no desire to play her as an origin character or even have her in my party when I inevitably replay this game. For such a great character this is an absolute tragedy, but since her endings are all functionally the same, there's nothing more for me to discover about her in different playthroughs. After having Shadowheart go down a good path, I'm looking forward to a Dark Urge game where she can
go down the dark justiciar/evil/embracing Shar route;
after ending the game with Gale in my party, on another run I can
have him sacrifice himself at the end and get the hero's death he deserves.
But with Karlach, since no roads lead to fixing her heart, one playthrough is sufficient to get everything out of the character, save a single dialogue option at the end that lets you choose death versus Avernus. On that note, I feel like there's no difference between choosing to die or choosing to go to Avernus--her problem is that her heart doesn't work, and neither of those address that. No matter what route we take, she ends up in exactly the same place we found her in at the beginning of the game.
Squid ending can stay because it's goofy as hell and while it feels bad on a character level (given there's no good ending to fall back on), it feels like an extremely out of left field D&D move I can absolutely see a player at my table making during a tabletop finale. Plus, if we had a satisfactory good ending for Karlach, I honestly feel like mind flayer would be a fine "option to save Karlach if you blew off all her personal quest stuff" kinda ending.
