I had the same problem for Headless Nick. My solution: Teleport the guy that owns the head out of sight and murder him. It gave me access to talk to the head
I also couldn't finish War of Stones, Trapped in Crystal (or wateva its called) and the Goblin head one, because Lawrence was already dead due to the another quest. And whats with the Watch is Coming quest? I've met the escaping Imps, but there's no clues about what you're supposed to do?
Would be nice if there was a way to force resolve these.