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.
Like I said, we disagree. At the dock the words that change her mind are "Zariel won't touch you" and similar comments from the PC. In that last moments her fear of death is greater than her fear of facing Zariel / Avernus with someone by her side. Before death is an abstract, in that moment it gets real. For me there isn't any development in her perception of life. And the epilogue dialogue supports that for me too.
And yes, I especially killed Gortash once more to take a look at all her bigger late game moments, and yes it is a heart breaking scene and yes she too has the empty moment after her quest is done, like Aylin, Astarion and arguably like Lae'zel after standing up to Vlaakith, Shart after defeating Viconia and Wyll after breaking his packt. For me there just isn't any new will to live or to fight for her life. In the version of the dock scene without a PC she feels very resigned and at peace with her fate.
I also realise that we mean very different things by internal conflict, I mean questions of conscience, what the character wants to do versus what they feel they should do, or two obligations that conflict with each other. Different forces that pull on a character and leave them at a loss about what to do. Karlach isn't really conflicted about anything, she has a problem to overcome but nothing that really challenges her perspective.
But I will leave it at that. Looking at all those Karlach moments in quick succession has made it so extremely obvious how differently the two suicide / terminal condition plots of this game are handled - that you are supposed to feel for the one character, while you are supposed to sacrifice the other without too many qualms - and I really don't feel like dealing with this any longer because it is really depressing.
