Actually the non-aggression pact came about as a result of Japan getting its butt handed to them by Russia.
China was somewhat of a sideshow for Japan. They wanted to conquer China or at least parts of it as a means of becoming an Imperial power and gaining respect from the West for being an Imperial power.
Their primary reasons for going to war were raw resources which Japan lacks. Primarily oil, tin, and rubber. The Japanese Army wanted to seize Russian oilfields in Siberia, even after its defeats by Russia, and the Japanese Navy wanted to seize the oilfields in the East Indies along with other resources there.
Unlike Hitler Japan wanted to carve out an Empire in the east and then sue for peace. As lopsided as it is I think the original Axis and Allies Pacific portrays this well with the Victory Points. The idea was to seize the resources in the Dutch East Indies and then build up a ring of outer defensive Islands. The Japanese knew they could not defeat the US and UK but hopped that they could make the cost of taking back what Japan had claimed too prohibitive for the Allies to undertake.
Also along historical notes I think China should be able to send units into Burma and FIC as they did do this in the Actual war.
I haven’t given any thought in Anniversary but I thought in Revised the Australian and New Zealand forces should have been US as they were under US command.