It seems it was drawn using the province borders of the Qing Dynasty (which is up to 1911). The current Chinese province of Inner Mongolia was divided in Republican times into three provinces: Ningsia, Chara, Suiyuan. Ningsia was definately controlled by Ma.
Anyway this ROC claims map seems to be the best representation of 1930s China in terms of provinces:
http://strangemaps.files.wordpress.com/2007/12/roc_administrative_and_claims.jpg
Sichuan was ruled by warlords till at least 1938 according to Wikipedia:
During the period from 1927-1938, Sichuan was in the hands of five warlords: Liu Xiang, Yang Sen, Liu Wenhui, Deng Xihou, and Tian Songyao.[...]General Liu Wenhui and Liu Xiang improved their forces, organizing a small airforce and an armored car force.
In in-game terms Sichuan would have a strong military but because it's internally divided it can be easily taken over by Nat China. The same goes for Guangdong, which was weakened by the death of Hu Hanmin. Chiang wanted make Guangdong more loyal to him. Chen Jitang conspired in turn with Guangxi to overthrow Chiang. Through bribery the conspiracy was undermined and in game terms, Nat China annexed Guangxi. This was in late 1936.
How to simulate this? It's quite simple. All the cliques are ruled by a Paternal Autocrat party known as Guangxi Clique, Shanxi Clique, etc. But the KMT is also a party in these cliques. By doing Support Our Party in the intelligence screen as Nat China, the KMT influence can be increased. When the Clique party becomes smaller than the KMT (or zero altogether) then Nat China annexes the Clique.
This solves a lot of the problems, I think. It's historical and it gives Nat China the boost it needs to survive beyond 1937 (more IC, manpower, leadership, infantry and an armored car brigade).
Regarding East Hopei, you might be right. I know it encompasses Tiangjin and part of Beijing. I also know it ends northeast of the Baihe River. It also includes the towns of Taku and Tangku. So I used your link which has a very accurate Manchukuo border to draw what I think where the borders of East Hopei in 1936. On a side note, East Hopei was absorbed into the Provisional Government of China.
Here's the Hopei map and below it an updated overview map: