Cain can grow back his head? I know Cain had this extreme showing once where he was able to survive as a skeleton -that was easily his best showing of healing but people take that as the average for him for some reason- due to his hate or some such keeping him going but even then he believed D'Spayre was about to destroy him.
Edit: I believe most of Juggernaut's power had been drained from him at the time. Muddies the water.