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.
They definitely had something, at least regarding the engine fix by some means (gondians and the shit ton of infernal iron), i don't know if they even had some ideas about avernus, but that would seem only logical tbh, it makes sense and it's referenced as you pointed out. In any case, I have the feeling whatever we were supposed to get was scrapped due to time constraints and not due them wanting to remove it all together because it was bad or unwanted. Or at least, as I said before, I HOPE SO. I also hope they are not gonna settle for these quick rewrites and rerouting of her character... She deserves so much more.
