As I have explained before, the reason Chinese movement is limited is to reflect the historical fact that China was also embroiled in a civil war (Nationalists versus Communists) at the same time that it was fighting the Japanese invaders. As such, it had little interest in matters outside its own borders (keeping the Burma Road open for Allied aid being an obvious exception). This restriction keeps China from influencing the game outside of its borders without having complicated rules to represent the impact of this conflict.
Don’t forget about the warlord factions!:) Sure they were nominally supportive of Chaing but he had to keep a decent force back just to police his ‘allies’.