Interesting question. It's easy enough to have MacArthur die before or during the war--personally, I have always wanted to see a TL where he is shot while trying to drive out the Bonus Army--but I have no idea who would replace him.
In the short term, I think anyone other than Mac would not have rudely rebuffed the Emperor's attempt to apologize for the war. That alone would likely have some interesting effects. There is also a decent chance you'd see the Showa Emperor step down in favor of someone else, as some in the Imperial Family themselves favored. If you had a replacement who was himself quite leftist, then things like the breaking up of the zaibatsu might have been persued with more vigor and thoroughness.
In the long run, though, I think the "Reverse Course" is almost bound to happen. Making a clean a total break with the past was a lovely idea, but it was a peacetime dream. Once it became clear that China was going to go Red, and certainly after the start of the Korean War, the US decided that it needed Japan as an anti-communist bulwark in East Asia. Working with people who had no ties to the Militarist regime was a luxury they felt they didn't have.
Really, I think the PoDs necessary to really purge all of the Militarist influence from postwar Japan all lay outside of Japan. A KMT-lead China, no Korean War, no Cold War... these are the sorts of PoDs needed. A tall order, to say the least.