Spoilers for Act 3

Spoke to the Mystic Carrion by breaking in, curious about the building, bought things from him.
Then did the Jannath House quest and went back to the Mystic Carrion to buy the cure.
He tried to make me do a quest, but I just paid the 3k.
Fixed the Jannath House.
Wandering around, found Thrumbo, he did not reference Mystic Carrion at all, had no clue they were connected.
Went back to Mystic Carrion to buy something, he attacked me, asking about Thrumbo, so I googled it and read up on the quest, decided to just finish it for Thrumbo and kill Mystic.
Get to the part where we discover the last jar is inside Thrumbo...but Thrumbo won't talk to me. He never gave me the quest, because I never accepted the quest to kill him, so I'm stuck. Mystic Carrion revives and I've no way to kill him, short of murdering Thrumbo in the streets, which both irks a lot of guards and defeats the purpose of killing Mystic.

I filed a bug report, but need Thrumbo to acknowledge that we can kill Mystic without Mystic sending me to kill him first.