I gave the 1941 scenario a shot as Communist China with the idea of seeing what happened at the end of the war. China collapsed and got puppeted in months. I'll have a look and see why.
Ah, it's because Nationalist China has 15% dissent, while Japan has none. Nationalist China has a horrible mix of half militia and half infantry units grouped into the same armies, so they all go at speed 3. Nationalist China has every unit at 50% strength only.
In short, Nationalist China has a number of massive drawbacks that Japan doesn't, and collapses too fast. Give it fewer divisions if you must, but start them at full strength. Take off their dissent penalty. Un-group the militia and infantry into their own armies. Remember that the China-Japan war is supposed to be a grinding stalemate until Japan gets nuked - the war shouldn't be decided in China.