Ill Met By Moonlight quest bug (who's surprised)

So the quest has bugged such that I've entered the grotto, but it's still in its old form and Sinding is nowhere to be found, there's just a quest marker on the rock where he should be.

I had been in the grotto before, and gotten Bolar's Oathblade and cleared it out and everything.

The thing is, I've tried every single solution that supposedly works: check that he's not still in the Falkreath Jail (he's not), disable and reenable him (I can agree to spare him and will then fail the objective "join the hunt" and get the objective "kill the hunters", but there are still no hunters and the grotto is still as it was before, even if I leave and re-enter)