I think the Occupation option is a dead end.
I personally don't think it's a good idea for the Soviet Union to integrate China into it, and there are a couple of differences between the possible integration of China and the real life examples you mentioned about Belarus and the Ukraine:
1 - Belarus and Ukraine were much smaller countries and easier to manage.
2 - They share the same Slavic descent with Soviet Russia and Russians thought of Belarus as basically the same nation.
3 - They used to be parts of the Russian Empire, which the Soviets emulated.
4 - The Soviets needed those countries to consolidate their rule in Eastern Europe.
With China it's totally different; not the same culture, a vast country, it would take decades to pacify. Plus, it would be far more useful to the Soviet Union as a powerful ally against the Axis instead of going through the humongous effort of integrating it.
That said, I might create an option to integrate China but with lower chance.
It is impossible to the soviet union integrate china, the chinese population in late 40s was over 700 million people, while the soviets union in the fatherland scenario have between 70 to 110 million people in their entire union, it is a hundred percent impossible to integrate a country in this situation, but I can see the soviet union annexing Manchuria and Mongolia, this two can be considered
I know these reasons, and I though about them.
Ok-not the oqqupation, but communist china obviously should be puppet-because to have this event you should beat mao zedong. And Mao always was in bad relations with Moscow, wether moscow had "28 bolshevicov" who were much more loyal than Mao.
So I thinc that they must be un power. And also-Integration of manchuria is still possible, as for me.
About full integration- I think, that the possibility should be, but there obviously will be high dissent, many rebels and will be needed a lot of IC (something about-25%)and resources- so, if the player chooses the way of unification of USSR and China there should be almost impossible for him to get part in WW3- and whis means that USSR will lost it`s european territories until the ww4- with USA- or even forever.
So player should choose wiesly-to make an ally which may rat in worst times- or to make a gamble and try to integrate with such wiered country as China with great trobles-but also with great profits in the end.