Japan never had a chance of holding China unless Japan would integrate itself into China. Much like the Manchus, Japan itself would disappear if they attempted such a thing.
Remember, Japan in the 1930's was at the peak of its power vis-a-vis China, fighting against a China *BEFORE* industrialization, still gripped in the turmoils of dynastic revolution in 1911, and fighting a CIVIL WAR to boot.
If Japan could only get 1/3 of China with those kind of odds, there would have been no way for Japan to hold Chinese lands after China has had a chance to recover. Period. You're delusional if you think otherwise.
The only way Japan could've held China would be like the Manchus, integrating themselves into a greater Chinese society. Which I don't think would've been a bad thing at all.
Also, I'm not even gonna answer the allegations that China was just chosen as an ally because they just needed a native representative. If that's what you think, then you have a lot to learn still about WW2.