Then it seems we broadly agree that the USSR can't effectively resist the combined might of Western Europe, even if the US is able to move some supplies through the Far East.
yep pretty much. Although how that much that becomes a post win quagmire will depend on what winning looks like
On the other hand I don't believe the Japanese will hold out for long enough against a US that is focused solely on the Pacific to enable the Europeans to come to their aid.
Thing is if the US focuses solely on the pacific what happens in the Atlantic / South America (in this TL the USN have more to worry about in the Atlantic than it did OTL)
I'd predict Japan surrenders to the US, the USSR surrenders to the Western Europeans, and a negotiated peace between the US and Western Europe follows on, because both sides will realise that neither can really effectively launch trans-oceanic invasions. The US won't want to try to invade Europe across the Atlantic and likely won't be very interested in trying to invade India either, and the same is true in reverse: the European powers won't be interested in trying to land in the Americas.
It might be that some of the more remote parts of the British Empire in the Pacific end up becoming American-controlled in a status quo peace. I doubt the Americans would have a serious interest in trying to conquer French Indochina or British Malaya, for example, because when they started getting closer to the Japanese Home Islands you'd start to see major forces being built up in those colonies. By then the USSR would be broken, or so close to broken that the Anglo-French forces would be able to spare the troops to put substantial garrisons in those colonies.
I agree with the others I think the US vs Japan & naval allies* is not the same fight as it was OTL. Also you have Japan not fighting in Burma etc (what's the deal with China in this TL?).
I think India might be potential though not by a direct invasion (at least not initially) but maybe by proxy war / incitement to uprising? I agree I think you see a cold war but that also means I think you see proxy wars. Indian nationalism is gaining strength, the US can play the whole "we used to be ruled by the English let us help you help yourselves". And frankly the Uk can't fight and hold an organised and supplied India.
*who still have their FE.Asian bases and colonies here.
I think Canada, like Ireland and maybe South Africa, would be neutral. British and other Allies submarines could operate from Newfoundland though, that had reverted to being a Colony by the late 1930s. And of course from bases in the Caribbean or British Honduras (modern Belize).
So you're right that it's basically the US Pacific Fleet that would be facing the IJN on fairly even terms. Unless or until the British and French send reinforcements to the Far East.
I think you are right Canada might well go neutral (although while the US could invade, if it does it's a tough bit of land to control entirely, maybe another proxy war)
S.Africa I'm not sure about, not sure why they would.
The Caribbean and Belize though I think they pretty much fall under US control and the Gulf / Caribbean becomes a US pond in short order!