I doubt Whitelaw would have felt up to it at his age - he was only a year off shuffling up to the Lords.
People forget how highly Francis Pym was regarded - he was mentioned as the natural successor during the summer of 81, people forget that Thatcher did not handle the riots well, and Heseltine's charisma on his fact finding in Liverpool got her out of a big hole. Pym was making veiled criticisms of the Thatcher economic experiment, and was a far more credible alternative leader than Prior or Heath.
Howe was ridiculed for his suggestion that 'the recession is over' during summer 81, and only Thatcher's support kept him on to make more cuts in late 81. Without Thatcher's support, Howe is nowhere at this point. Pym became foreign secretary when Carrington resigned, I think he would certainly have been the leading candidate for PM if Thatcher had gone, as the Thatcherite economic policies didn't yet have any proof that they would work.