After the screenshots, I managed to take everything to the south, only a couple of western provinces left for Nat China. Of the states infiltrated, I fully controlled Henan and Xian.
And the peace conference? A big "f**k you Mao"...a massive 17 points! Which resulted in getting Xian on the first round, after which Japan gets to puppet everything else.
In conclusion, don't bother grabbing land, just declare the uprising and sit back and grind some war score. Anything else is a waste of time. It's is actually more useful to annex Shanxi, Xi Bei and Sinkiang before doing so, so at least you get the few factories from them...
that would be doing it wrong. Why did you infiltrate China when Japan was fighting them? Ofc you're getting very little.
You infiltrate Japan when they're fighting China. If you're in your own fight against them (at least not the same war as with China) you may even get them to give you the 'all mainland for peace' deal and leave China with nothing.
Then infiltrate China afterwards.