Speaking of Dai-Nippon... wouldn't they be eventually in conflict with the Russians? In the novel, I figured that the major rivalries would have also included the traditional Russo-Japanese tensions over Manchuria and Sakhalin, among other things.
Firstly, the Japanese have almost NEVER gotten along with Russia, and they always seem to wrestle for Manchuria, Sakhalin and the Kuril Islands in any period of history ( in OTL, they still haven't figured that out.)
Secondly, the region of Manchuria would have been a vital resource in Post-Fall reconstruction. Given Russia's closer proximity (i.e. an actual land connection to Manchuria and China proper), Russia would move in once it managed to figure out whether or not it would eat itself, and by then, around the 1890's-1900, the Japanese, who happen to have survived intact, might have already occupied the region for years. Could a Battlefield 2142 scenario result, where the Russians would invade for the prospect of life-saving resources? Either way, it seems that a Russo-Japanese War, in some form, would have happened around the same time and circumstances of OTL.
(Alaska, maybe?)
I'm assuming Dai-Nippon has included Chosen sometime in the Post-Fall chaos.