For me she goes back because someone comes along and she is maybe more afraid of death than of Avernus when someone is by her side. If you sacrifice yourself as Gale and get her parting comment, it's something like "See you on the other side." Going down the dark street with a friend feels saver and more doable then going there alone.
You say that, yet every time your MC proposes that you go to Avernus together, she flatly refuses you. She doesn't even consider it. Until the scene at the pier.
She doesn't really fear death, because she's ready to blow up without much qualms if you let her. She gives her goodbye to the world, and then she bursts into flame. (Heck, she even refuses Withers' proposal to resurrect her)
Like both of us said before, if it was only about companionship, she would've taken you up on your offer long before the final scenes.
The only way you can explain why she constantly refuses you yet agrees at the pier is because she's internally conflicted about what to do.
I don't know how much you remember about her rant after killing Gortash, but that rant was the first time her facade broke. It's the first time she admits to you that she feels envious that you get to live on and she doesn't. She even sarcastically asks if you have a Wish scroll on you. She laments the fact that the heart her mother gave her was stolen. All of it is about how she wants to live. It's a very selfish, vulnerable, and human outburst that you wouldn't get if her conflict was purely external.
Before that, she was very adamant about not going back despite your protestations, yet she agreed in the end. That can only happen through introspection.
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It just bugs me, that there are still characters who cannot allow her to go to Avernus with Wyll, which means the player has to sit through a rather painful sequence of her blowing up for absolutely no good reason.
I absolutely agree. I don't know how difficult it is to sequester characters' scenes but it should absolutely be at the top of the list of things to work on. Characters shouldn't be sidelineing each other's scenes unless they're mutually exclusive, which isn't the case here.
What's more, I think Wyll shouldn't even be tied to Karlach at the end there. If you or Wyll don't go with her, there should be a way for you to convince her to go with someone else whom you haven't romanced and whose storyline doesn't necessarily conflict with hers. Minthara has by far the strongest lines on that topic I have seen from any character. Even Ascended Astarion sees the challenge as a tantalizing prospect. Surely it can't just be Tav and Wyll.
