Hm. Is there ever a Japanese-Soviet war, or does the USSR just keep the China war "hot" with military supplies, secure refuges, and advisors until the last Japanese officers are helicoptered out of Peking?
OTL experience would seem to argue that at some point the Japanese leadership will be able to argue itself into believing that with their new anti-tank weapons and superior air force, they will be able to cut off the trans-Siberian, destroy Soviet forces in the east, and force the Soviets to stop aiding the Chinese.
Who knows, perhaps we'll even get Vladivostok in the peace...(That, of course, would be the "Soviet land grab" scenario.

)
On the other hand, perhaps the leadership turns on itself in another period of assassinations, and after the radicals are bloodily purged, the Yamamoto government faces the music and pulls out of China...
Not sure that the US would necessarily be the first to develop the bomb - in a no-WWI world, the Brits still have a lot of commitments to worry about, and OTL Tube Alloys is contemporaneous with the Manhattan project. Perhaps they develop the bomb as a side effect of trying to develop nuclear power for their navy (IIRC, building a nuclear reactor is fairly cheaper than building an atom bomb for the first time)
Bruce