i myself imagined that this rollercoaster was due to rewrites and leftovers from a time when you could actually do something about the whole situation. Emotional whiplash indeed.
Speaking of rewrites and leftovers, I just remembered something she says at camp after you kill Gortash. She says something like "after I die it's between me and the gods", and you can tell her, "No, it's between you, me, and Zariel, in Avernus". She laughs it off as being ridiculous, but the fact that the line was there and the epilogue wasn't patched in until later makes me wonder if they had a beefier questline planned for her where you go to Avernus and solve things for good. They even wrote in a way for you to get to the hells with the diabolist and stealing the hammer. Maybe they realized it'd be too much extra or just didn't like the idea and scrapped it.
