How does this work exactly? Would the B29s not be dropping over Berlin...?
Yeah, I had the same early thought. There are lots of variations that make this not the assumed Euro version of Nag/Hiro though. For instances, an Eastern Front won by (lets use the latest OP year) 1943 likely involves more than a few occupation forces battling partisans and red army remnants (assuming Stalin and the party move past the Urals), but in this case Fortress Europe is going to be insanely more difficult for any mainland landing. Forces diverted are going to have major impacts on OTL situations in Europe and the Pacific. I would imagine major discussions debating whether to keep a 'Europe first' policy happens in the White House too. But do the Allies still attempt Operation Husky (Sicily)? If not , or if the Russians are knocked out before, are forces released to North Africa to quite likely renew pushing out the Allies and starve off Malta? If through preventive offensives or through counter offensives in Sicily, the Allies are systematically pushed largely out of the Mediterranean theatre that has to have major repercussions on any future Allied landings. Fortress Europe is going to actually look like a fortress with major combat forces available on land an in the air. Perhaps not some of the more fringe systems come to be, but I would imagine after victory in the East, major resources and advances in Luftwaffe tech, numbers, pilots, and weapons sees much earlier deployment dates. Lastly, with Japan still very much in the fight, coordinated efforts to keep her in the war will take place through some of the changed ATL situations. More resources available to send or offer, more routes of assistance and coordination opened through North African (Suez canal) victories, Japanese offensives through an opportunist DoW, or just a much reduced British presence in the Pacific theatres through losses in the Med or pull back to guard India's western borderlands.
All of that, is seemingly to paint the picture that an OTL Pacific war is not at all likely. For the sake of the OP questions, keep that in mind for any kind of post-war China and USSR. Let's run with the notion that some kind of US led allied war effort to defeat Japan does take place. With or without nukes, Japan is occupied or isolated. Americans see a far larger and more lethal threat in an undefeated Germany now, so all priorities have shifted to strengthening Atlantic. The UK probably has more Americans on it than in the Midwest. That means insanely increased funneling of guerrilla and conventional arms to whatever is left fighting in Russia. I can't imagine Nationalist China being abandoned and with or without Mao accepting, the US probably doesn't want to openly slice up Soviet territory, so perhaps offers are made to let Chinese manpower under protection of American air and naval forces, occupy Siberian territory (with behind the scenes winks for permanency). If Chinese forces are stable enough, I tend to think this would be as much land as they could handle. If some centuries old map of Chinese claims could be found, it might roughly resemble that.
Without peace in Europe between the US/UK and Germany, I don't think the allies 'officially' allow annexations. On the ground they would probably welcome the Chinese in, if that provided more power than some kind of rump propped up Russian resistance government or state. If we presume that victory on the Eastern Front precipitates negotiated peace in Europe in some form, then perhaps Americans are more willing to allow some kind of major Chinese land grabs to prevent German influence extending to the Pacific.