I've run this several times now on each difficulty, but last night near the end of a run on Hard, the planescaller died and we got the old "Quest Failed" dialogue. This was the same dialogue you get on part 1 i.e. "The planescaller falls to the devils..." if the devils kill her, not the new dialogue you get if everyone exits the last part at the same time, the "With no one left to protect her..."
We were over 200 kills into the last part of the quest, and the whole party was still alive and fighting. I think I had 50 or so devils charmed. AFAIK, this shouldn't happen in the new patch. I've bugged it, but I'm curious if anyone else has seen this, or if there's any clue why this might happen.