Regardless, I see a few obvious things:
1. Japan is either a) more sane but with the military in charge
Or b) with Taisho Democracy
In a), the GMD would definitely win (without an invasion).
In b), that really depends on relations with China. If they're terrible, Japan might choose to fund the Communists, instead of supporting an invasion.
2. China is stronger, probably. In my timeline, I'm thinking about turning China 'red' through a worker revolution, instead of the normal 'Maoist' revolution. That requires industry.
Perhaps Chiang feels more confident to destroy the warlords after the CCP is 'gone'. Then he establishes dictatorial control over the country, using his landlord-supplied army.
This gets pretty unpopular with the peasants - and bang, Maoist revolution.