"The Mark would need him to be alive. It just needs a living body for it to be a parasite to. It wouldn't be out of the question for it to kickstart Dean as a demon if it's what it took to get his body moving again."

It is all very complicated and it should be remembered that Lucifer is the sole creator that crafted that revolting mess.

An affront to humans, an affront to angels.

... Lucifer was extremely angry when he made demons, clearly.

"Well." What does he say to that? That's Istredd's weak, then? But no, maybe if it was the reverse, Lucifer wouldn't do so hot either. Call him a monster with the mental chords to back it and see what state of the devil came out the other side. The bond would make a scenario much worse in that regard, I suppose.