Prevent the assassination of Liao Zhongkai.
Probably. The rift was largely put in place by Chiang's orders to perpetrate the 1927 Shanghai Massacre, and I can't see anyone else being dumb/ruthless enough to go through with it.
You should ask the question: If the CCP didn't go around subverting everything it touched, would China have resisted the Japanese more efficiently?
What do you mean?
If th CCP and the Kuomingtang cooperated more, would they be more able to repulse the Japanese?