On the occupation: I'm not sure, I feel the problem was more who was made "Shogun", i.e. an incompetent but politically powerful prima donna of a general, more than the length. They should have organized things such that it was actually possible to form a civil service.
As for Hirohito, I could easily see Britain protesting that last one; of course the British are, at that point, as dependent on the US as the french for economic survival for the next few years so they can't really do much. And of course there's the problem that a republic would not fly at least with the Japanese elites (I'm not sure the people would have cared as much compared to other more pressing needs, like eating at all). With Akihito only 12 at the time they did have a clear successor who'd be an adult in a few years and was mostly clean thanks to his age though.