Do you have any mods installed? Given the steps you've already taken in attempting to fix it, I'd say your best bet will be looking through your mod list, and look into any new mods you have installed and disable them and load the save again. Trial and Error really is the best way to fix it.
If you don't have any mods, it's simply a bug--one that you've already attempted to fix with no avail. So you'd likely have to either start a new game in order to do the quest, or just play the game without touching the main questline.