It's ridiculous to compare this to colonialism. Colonized nations didn't go on a rampage across the globe before being colonized.
Japanese society pretty much need to be rebuilt from the ground up to uproot the fascist strain that lead it there. Just look at OTL and how they still honor soldiers of WW2 and refuse to recognize their crimes in Korea if you want to see what happens if you don't.
Plus its socialist movement need time to get its shit together considering how long they've been underground.
Not entirely true on point 3 - IOTL, the Japanese Communists actually came out of the war in an extremely strong position because they were the only group that had actually spent the entire Imperial period in prison and had made no concessions to the regime. And the Socialist Party, while more moderate, won the first elections. While the latter screwed the proverbial pooch upon gaining power and the former ended up a casualty of the rising cold war, they were really the major mass political forces in post-war Japan. It's similar to what happened with the Communists in Italy and France - marginalization, along with a popular economic line, made them immensely powerful. The Communists in Japan are still the world's largest communist party in a liberal democracy by membership.
As for the whole occupation thing more broadly, it's not too far off OTL. Consider that Supreme Commander of the Allied Powers under MacArthur and then Ridgeway ran Japan as an effective dictatorship from 1945 to 1952. In this role they unilaterally rewrote the entire constitution over the wishes of the Japanese elected government, making it one of the world's most unabashedly progressive. They read and censored every piece of mail, every book, every radio broadcast. The Americans re-isolated Japan, basically forbidding all foreign information and travel into and out of the country. It was forbidden to talk about SCAP. SCAP built a welfare state and planned the economy. It determined cultural policy. It overhauled the land system and effectively ended Japanese rural poverty - a revolutionary step by a lot of measures. It built a welfare state from nothing and imported American union practices. By a mixture of repression and reformism it restructured the economy and broke the momentum of the left. It - briefly - conducted a wholesale purge of the old elite. It preserved ththe Imperial house against real apathy among the masses and hostility among segments of the elite. And it did all of this with zero democratic consultation as an effective military dictatorship. Even after the abolition of SCAP, the America armed forces remained a key direct political actor in Japan through the 60s - some of the worst instincts of the Japanese establishment were born in the desire to preclude a mass American political crackdown. Regardless, I think people underestimate how wide-open Japan was in 1945 - pretty much all the basic facts of modern Japanese politics, from the continued existence of the Imperial house to the economic and social structure, were determined then.
So within this timeline, whatever Japan gets won't be much different than IOTL, except the VladPact will probably just be more likely to install the Communists, Socialists, or an American-style United Front thereof as their chosen party rather than the OTL conservative-reformist coalition - something that probably would get general popular assent.
The real kicker is that here we're presumably not getting a Korean War, so the impetus to end reforms, come to an understanding with elements of the old elite, and reindustrialize just wouldn't exist.