This thread caught me in the wrong time (I was moving out of a dorm)... but it still shouldn't have taken me a month to reply. Note to self: use bookmarks.
Anyway, I have now looked into that save. I don't fully understand what really has happened there, but I think we have two bugs interacting here: the Autosave bug of the engine and a quest bug of MQ17. Arantheal's "state" variable is down at 12, while it should be somewhere around 23. The dialogue between Arantheal and Barateon at Zelara's grave (yet before the Soul Flayer temple) alone should have brought it up to 18! Either some things went wrong when patching, or it's the engine bug that forgets modified values of variables when loading an autosave. As for the quest bug: Apparently you have killed some of the soldiers besieging the monastery on your own; laudable as it is, it seems to have broken MQ17. Here is an ESP patch that fixes it. To fix the autosave-induced problem, click on Narathzul from the console and enter "set state to 23". Then, wait 10 seconds (you should get a quest popup message). Narathzul will seem to die in his own blizzard spell, but disregard this - he will be right back; he is essential.