It's been a while since I read the manga, but did Mukuro die? It's got me all nervous since I got the mist ring. Was that just the future him? Is his past version safe in the prison?
If they manage to kill Irie Souichi, then the past will be reset and set back to normal, then Mukuro should be fine (at least as fine as he can be in that prison). As far as Mukuro being dead at the moment, they haven't said so far. We've only seen that he's been personally caught by Byakuran himself. Mukuro is just presumed dead because of the Vongola Hunt (Kill on Sight kind of thing).
