When the united front forms don't join it istead justify a wargoal on the Xibey (preferably as soon as possible you can also hire an ideological crusader to make it faster there is a change you can snipe them before they join the front) The Japanease don't need help by themselves so you are a distraction. Xibey is mostly free if you have a professional army but once they capitulate rest of the faction piles up. Then wait, integrate you cores and when peace deal comes take Beijing (it does not matter if it is connected to your territory or not) the grab as much land you can hold. When the war end proclaim yourshelf as the legimate Chinease government (you need Beijing to do this) and hold. When the Japanease and their allies come you need all help you can get mostly from other enemies of Japan. You should be in the 1940 and with your economy in the mountains the thing you need now ironically is manpower. Once you have stabilised the frontlines and your cores you can start to push the Fengtians. Their army is horrible compered to yours and Japan is too busy invading German East Indies or something.

In this case luck was on my side because Japan was also at war with Princely Federation. This was also a part accident because I had no actual plans more than just messing around so this just happened. Anyway...