Sorry this just doesn't just not make sense, it's straight up wrong
Japan treated the 80,000 Russian POWs well in the 1905 Russo-Japanese war and was commended by the International Red Cross for its treatment of Russian POWs. This was apparently driven by Japan's desire to be seen as a nation equal to western powers. In the previous 1894 Sino-Japanese war, Chinese prisoners and civilians had been treated rather badly and massacred at Port Arthur. A fact which the Japanese Army and government obscured for it's own internal reasons as well as negative international opinion. 5,000 German troops captured in 1914 also received good treatment and had a low death rate (approx 1%) for three years of captivity.
Japan's treatment of it's own captured soldiers and sailors for those three conflicts is a more complex story. The ideals of the Japanese military emphasized death before dishonor in all of it's wars, but their policy was often flexible. In Japan's civil wars prisoners were either executed or pardoned depending on political expediency. In the Sino-Japanese war, propaganda was produced demonizing the Chinese for cruelty and warning troops against surrender, but almost no troops (only 11, 10 porters and a soldier with a head wound) actually found themselves in a situation where it would have been required to surrender.
In the Russo-Japanese war about 2,000 Japanese troop were captured during the war and repatriated at the end of it. These cases were all investigated by postwar military councils to make sure they did their full duty before being captured. Soldiers could expect to have to apologize and endure insults, but were seldom harshly punished, and only a few officers were forced to resign or cashiered. However, informal administrative punishment could be very unpleasant, and out of the service some former prisoners were ostracized by their communities and forced to move elsewhere. Stories circulated regarding the disgraceful fates of dishonored soldiers and their families and these set the stage for the later official and unofficial prohibitions against surrender in the WWII era.
Source: Drea, Edward J. Japan's Imperial Army: Its Rise and Fall 1853-1945
If Japan has control over Manchuria, than Chinese Central Authority does not exist. And besides, Japan has the power to disintegrate the whole damned country. In OTL they managed to make successful offensives against China as late as 1944, in TTL Japan would just obliterate China in a war.
Honestly I see Imperial Japan having a good chance to win the Cold War, I already said why Germany would become irrelevant by the 70s and now i'll talk about why America will collapse.
Personally, I think they could keep ahold of Canada easily enough because well...All you have to do is swamp the place in Settlers and you outnumber the people there. Not to say that the Candian Population would be irrelevant but managing to dislodge the Americans? Nah. As for Utah, they answered that in probably one of the Cringiest scenes of the books. Flora Hamburger got to pushing a plan of whats basically Ethnic Cleansing and deporting all the Mormons to Hawii (And this is depicted as a Good Idea, which also because it comes from Flora "concieus of the House" Hamburger makes it pure cringe). But on the Confedracy? Yeah, I dont see it happening. The South has had too long to develop a indepdent identity and unlike Canada, you cant just drown them in Numbers. You could maybe make it work if you took a more soft handed policy to Military occupation and focused on making peoples lives as comfortable as possible and educating the next generation as 100 percent Americans but well... stuff like killing 1500 people for the death of 32 sailors is not making any freinds. Thus it counts on the political will of the American people to maintain the Occupation and after the first Remeberance Generation thats going fade fast.
We see US has Philly superbombed by Potter. Newport News was superbombed by the Americans. The former CSA is an economic ruin after a devastating war and a genocide, Canada had a three year rebellion that hindered the US war effort, the Mormons rose up (again). At least half a dozen or so states experienced active fighting and widespread destruction. I expect the US to be very sort of draconian especially given how they treated people in Florida post-war.
I don't see Canada getting self-determination. The Democrats are the Party of Remembrance and won't give up power to the Canadians, the Socialists are tarred by the Richmond Agreement and will be unwilling to risk another blow-up in their face, and the Republicans have been out of power since 1884 and are the point-five in the 2.5 party system the US has. With the Democrats unwilling to compromise, the Socialists tarred by their last attempt at compromise, and the Republicans unable to do anything, the situation will just inevitably become untenable while the US gets in a Cold War-esque scenario with the Germans and the Japanese.
The US is, as far as I see it, a Prussian-style USSR which will inevitably collapse under its own weight while it was suppressing the Mormons, the Canadians, and the former Confederates.
The ratio I've generally seen is 1 soldier for every 50 civilians for effective counter-insurgency. And while have no idea what the population of all the US occupied territories will be by 1990, if we go by OTL's 1990 population numbers that will give us some idea of how big a military the US might have to maintain just to hold onto those territories.
The Southern states (including Kentucky and Oklahoma but not including Texas) have an estimated 60,611,407 people, so that would require 1,212,228 soldiers.
Canada (not including Quebec) has an estimated 20,793,014 people, so that would require 415,860 soldiers.
Utah has an estimated 1,722,850 people, so that would require 34,457 soldiers.
Cuba has an estimated 10.6 million people, so that would require 212,000 soldiers.
The 2 Baja provinces, Sonora and Chihuahua have an estimated 6,244,098 people, so that would require 124,881 soldiers.
And Hawaii has an estimated 985,000 people, so that would require 19,700 soldiers.
So if all those places still require military occupation then the US would need an army of 2,019,126 soldiers just for occupation duty. And realistically the U.S. Armed Forces will probably have to be at least twice that size so that the U.S. can have a decent navy and air force and some reserve capability. As such we're probably looking at a military of at least 4 million people. The U.S. probably can just barely afford that but it will mean accepting considerably higher taxes and a greatly reduced welfare state from OTL.
There's a non-zero chance that we see both Germany and America collapsing and a Imperial Japan ascending as the world's hyperpower, drawing all nations under the sun under the co-prosperity sphere and the twenty first century being the Japanese Century.
Which might make for an interesting grimdark timeline as Japan brings the Co-Prosperity system to Europe and North America and gradually crushes all hope for the future through crushing mandatory economic specialisation and stripping the former Imperial core of the world for wealth like Britain did with India and China in the 19th century until Europe and North America are driven to poverty to elevate Japan's prosperity to unmatched heights.