So, the basic PoD in this idea is that the Nationalist forces begin to extend control over China much sooner, leading to a conflict between Japan and China in an attempt to ensure Japanese hegemony over East Asia. As in OTL, the US ends up intervening, and a bloody war is fought between the two nations in the mid-20s. The peace agreement forces Japan to yield some of her colonies (not sure how much would be realistic), nominally abandon imperialism and democratize significantly.
When the Great Depression hits, this fledgling "Japanese Republic" nearly collapses. The Japanese Communist Party ends up becoming the strongest party thanks in equal parts to the success of the Stalin's USSR through the Great Depression and the disgrace of the rightist factions from the war. Perhaps a major constitutional reform happens as well at this point, ensuring the principles of Communism remain a part of JCP for the indefinite future.
So how plausible does this seem?
When the Great Depression hits, this fledgling "Japanese Republic" nearly collapses. The Japanese Communist Party ends up becoming the strongest party thanks in equal parts to the success of the Stalin's USSR through the Great Depression and the disgrace of the rightist factions from the war. Perhaps a major constitutional reform happens as well at this point, ensuring the principles of Communism remain a part of JCP for the indefinite future.
So how plausible does this seem?