I've searched several times, and checked my "completed" quests. No I haven't done this quest on this character, but yes I have all other deadric quests done.
The only thing I can think of to explain why he is gone, is that I did the forsworn investigation (minor spoilers). At the conclusion of that jail type quest, they reak chaos in markarth killing several. I feel like he's a quest npc so shouldn't die, but either way I can't find him, so maybe a forsworn killed him during that battle. Really don't want to start a 5th character for this one (rather large) echievement. And no I don't have a load to before that quest, that was roughly 70 (in game) hours ago.
Any other ideas/solutions?