Who takes over depends somewhat on who takes the blame. I assume it would be Carr, but Whitelaw is Northern Ireland Secretary. Even if Whitelaw avoids the immediate blame, he'll be greatly tarnished because of his dealings with the IRA Council the previous year. Maudling's career has been finished for a year by this point, and Thatcher is a nondescript middle-ranking (female) member of the cabinet.
It's hard to see who would win at this point. Powell would give it a shot, but if he does, then the establishment will be more likely to coalesce around a safe pair of hands, and nobody would seriously want to put someone like Powell in at such a point. I wouldn't rule out Douglas-Home being parachuted in as a caretaker until the next election. The butterflies here are probably sufficient for the Tories to win the next election, but I wouldn't put the house on it.
I don't honestly think it would have the apocalyptic impact on Northern Ireland people always seek with these kind of PODs. Internment and the tough approach of the early seventies will go on longer though, making a settlement as elusive as ever.