On the topic of China and the Great Powers, have you done anything to dissuade Britain from launching massive land invasions into Tibet and Yunnan and annexing large parts of China?
With China broken up into substates (even though they will help each other in any war, I doubt they will be as effective) this may only get worse.
This didn't happen historically, largely because China and Tibet were cut off from Burma and India by massive mountain ranges and thick jungles, with only occasional mountain passes or jungle tracks. Yet in Victoria 2 it is not unlikely to see Britain send 100 000 soldiers through these areas and into China, which would have been impossible historically because of the terrain and disease. Even in WW2 this proved a very large obstacle.
Perhaps increase the attrition these areas have (and give the AI a better understanding of attrition) or simply make the areas impassable.