I know in the original Hoi2, an event triggered in march-december of 1936, that could either prolong or stop completely, the Chinese civil war. Not sure how DH works. In the original they could choose an option that would basically prevent Comm. Chi from doing much of anything until the defeat of Japan. I haven't tried out Com. Chi. in DH, but if anything, *DONT* wait on the civil war to play out historically, because if anything, it most certainly will not play out historically, if anything you'll likely perpetually be at war with Japan - or at least much longer than historically, leaving you precious little time to spread the glory of socialism to all of China.
IRL the CPC attempted to take out the warlords which were immediately to your west - this is often difficult in the game (Hoi2) as the nationalists would annex them whenever they lost VPs, but there is a way around it. IRL they failed and lost many troops, but this is Hoi, not real life. It's entirely possible (in hoi2), if you're fast and a bit lucky, to take out every warlord in China, and then DoW the Nationalists when the Japanese invade, and attempt to grab as much territory as possible. If you're lucky, the Japanese won't even declare war on *you* right away, leaving you a free hand to deal with the Nationalists. Then it's on to war with Japan, on a foot which you can defeat them, with a bit of skill and luck.