Free chinas not getting nukes anytime soon unless someone out right gives them to em. OTL they didnt get them till the late 60s and thats without them losing half there damn country and without the civil war lasting significantly longer than OTL as it likely will in this TL. And Stalin hated Mao and certainly isnt gonna risk starting a god damn nuclear war to save him
I agree Japan is going to have a problem. Consider this China today has about 1.4 billion and that is with Mao birth control policies in place, the Japanese Empire has about 350+ million people, I estimated and no-one disputed that figure. So we are looking at about 4 times the population.
Now let me do a very simple first level approximation of Japan's empire GDP
Japan/GDP per capita today 38,894 USD
Japan GDP = 38,894 USD x 350 million = $13.6 trillion Hold this thought
Scenario 1: The communist come to power.
I am not sure what the US would do if China went communist, Japan vs Russia is a difficult choice for them. Japan would have an advantage here of being a huge market which would be very attractive to many in the US and some of the US allies like Australia assuming the Japanese do not try to expand. Mao is as you state much weaker here. One problem here is that its hard to imagine that Japan would get sit there and allow a red government under Mao form. If it did and the Chinese do as well as the OTL
Today China/ GDP per capita 8,123 USD
Now if we take $8,123 x 1.4 billion = $11.4 trillion is China GDP
China has 4 times the population and approx the same GDP but....
Scenario 2: The Nationalist come to power.
The US would like in Tawan support it. The Nationalist were very good economic managers and I would expect that China would quickly recover.
Taiwan/GDP per capita 31,900 USD
Now if we take $31,900 x 1.4 billion = $44.7 trillion is China GDP
China has 4 times the population and approx the 3.3 times its GDP
The Japanese Empire is in serious trouble if China here does want the land back plus a rich and democratic China (as Tawan is today) here would be very attractive to the Chinese under Japanese rule.