Figured I'd post this here to warn other players, seeing how in-game support must be on permanent holiday.
At the end of Devil's Details, Hathera now respawns if you bring her all the way up to the shrine room. She respawns with full HP and is no longer targetable, therefore the quest is not able to be completed.
I posted for in-game support, but after waiting two hours inside the quest for help - my 30% pot ran out, thanks for that - I ended up leaving.
Oh, also please just remove the in-game ticket feature, as it's clearly not monitored any longer.