So I got a radiant quest that sent me into Morvunskar long after I'd finished the Sanguine Rose quest line...and the instant I came in the door, I heard fighting downstairs...when I worked my way down there, I found that Sanguine had spawned at the top of the dais and was stuck in a run animation, while all the NPC necros were trying to kill him.

Amusing, but probably not intended?

Someone posted almost exactly what you have a while back and the thinking then was that Sanguine spawning there was one of the fixes from an earlier patch to stop from happening the bug where people were getting trapped in his realm where you complete that quest line. Who knows if that's actually why that happens but it is amusing I agree. I'd just chalk it up to the wackyness of Bethesda games such as this one and move on, heh.

That's how I understand it. Sanguine is there as a result of the 1.4 patch so you're don't get trapped in Misty Grove. Morvunskar has always respawned with the other NPCs though.

The best part is that Sanguine uses generic beggar dialogue when idle or in combat.

"I'll teach you to pick on me!"
-Sanguine, as he punches a necromancer across the room

well if they had to have him respawn, they could always have disabled him if the questline was done...odd they didn't think of that...

He isn't set to respawn, but I guess a script keeps him there..