I think people are overlooking the Avernus with Tav option. Knowing Zariel in DnD lore, she would accept them both as Champions, and they could fight the Blood War together. While this seems like a cruel fate for Karlach, from her dialogue we can see that she didn't run from Avernus because she was a 'slave', she ran because she was alone, having her chance at love and connections stolen by being stowed away into a realm of heartless Devils (enforced by her Compassionate approval tag). She had respect and titles (Demonbane), Mazora was jealous of how much Zariel favored Karlach over others, and Karlach admits that she enjoyed fighting demons (further enforced by her Violent approval tag).
It would be very in character for Zariel to forget about Karlach's insubordination should she come back to her willingly, and pretty much guaranteed with Tav offering themselves as another Champion (A base level 12 character with direct patronage from a Lesser Deity is extremely powerful, even more so in the Deity's own realm of influence). It is bitter sweet, and horribly resolved in the actual game with no sliders or post credit scene or quick in-game battle against Demons surrounded by friendly Pit Fiends etc., but Tav and Karlach continuing their lives and romance as Champions of Zariel is not a bad fate by any means, and would cure Karlach of the loneliness that ate at her in Avernus. The main issue with this ending is the execution, the Soprano's style fade-to-black with no shown or implied resolution. For all we know Zariel immediately kills Tav and forces them into a soulcoin to punish Karlach (very unlikely, Zariel puts protecting the multiverse from the Abyss above all else) or they arrive in Avernus and immediately run for their lives being hunted like rabbits. The resolution isn't there, their fates are left to interpretation which in other stories can work, but doesn't work here.
Now I'm not saying that she shouldn't have a more difficult resolution to her quest that sees her live happily in Fayrun alone or with a romanced Tav, she should. Returning to Avernus with Tav should be an outcome from a quest-fail state combined with a successful romance, not the best outcome. However it's the best outcome we get, and knowing DnD lore, their fate would not be a terrible one. They get to have their fairytale romance in the downtime between fighting in the Blood War. We are just not shown this, and Larian failed to resolve the storyline they implemented with poor execution rather than poor ideas.
I did cover the Avernus ending (somewhat extensively, with options) in my original post. Fact of the matter is for the entire game (until that end), Karlach is adamant in not only her refusal to return to Avernus, but also returning to Zariel's servitude as the 'Fury of Avernus'. That simply wouldn't be any shade of positive ending for the character. Best case scenario, Larian forges a good ending from the pieces left on the table in Act 3, and allows us to fix her heart. But second to that, the House of Hope, Doomslaying through Zariel's forces and demons in Avernus, some kind of positive epilogue. But certainly not serving Zariel in any capacity.
