Canada and Utah won't take much effort to keep or assimilate. The Confederacy would take more, but keeping and assimilating it is doable.
In 1914 Canada had a population of 7,879,000. Subtracting Quebec's 1911 population (I couldn't find the 1914 number) yields 5,873,224. In Filling the Gaps, Craigo suggested that 15% of Canada's population was mobilized in the First Great War. In OTL, about 10% of Canada's mobilized soldiers were killed. Assuming that a similar percentage perished in GW1, then the total goes down to 5,785,125. I wouldn't be surprised if the actual number killed were even higher. In the OTL 1920 US Census, NY state, Pennsylvania, Illinois and Ohio all had populations bigger than 5.75 million. NYC alone had a population of 5.6 million. Using the standard rule of one soldier for every fifty inhabitants, Canada needs only about 115,000 soldiers to hold it after GW1. Starting after 1917, anyone who immigrates to Canada (from either south of the 49th parallel or elsewhere) is unlikely to be sympathetic to any Canadian Independence movement. Over time, the population composed of those in Canada before 1917, and their descendants will decrease. Eventually military rule in occupied Canada will end as the former provinces are admitted as states. Once the Canadians have statehood and citizenship, I would think that all but the most extreme Canadian rebels would put down their guns. There is also the fact that post GW2 no foreign power will support Canadian independence. I am not aware of any insurgency that succeeded without foreign support.
In TL 191 there are at most
250,000 Mormons by the end of GW2. They aren't going to be a problem in Hawaii (since they have no weapons and no foreign support). Also, I don't think many Americans would oppose the forced relocation of Mormons. In OTL, few opposed the internment of Japanese Americans. And while Mormons aren't the victims of racism, the US (both in OTL and in TL191) has a long history of anti-Mormon prejudice. Also, Mormon extremists did use suicide bombs against civilian targets. Mormons will have few advocates among Americans.
The Confederacy, being the most populous of the three, would certainly be the hardest to integrate. But after four destructive wars, few Yankees would be interested in allowing an independent Confederacy. So there would be an occupation of the South. How successful would a Confederate insurgency be? Probably not very. The rebels would have no foreign support an the only weapons would be those left over from the war and those they can steal from the Yankees. How much support would they have? I doubt much. In 1945-1946 the Allies treated Germans absolutely horribly - uprooting millions, economically exploiting the country, etc. The treatment of Germans was probably overall worse than any treatment the Southerners would get. There would be reprisal killings, sure, but would there be ethnic cleansing that would kill hundreds of thousands? If the Germans didn't rebel, why would the Confederates? At best I could see something like the OTL Forest Brothers, and like the Forest Brothers, they'd be defeated by the early 1950s at the latest.
Now, I suppose the Yankees could decide to go all Morgenthau on the South, or even worse. But that seems unlikely. Since there goal is to annex the South, destroying its economy and people would be beyond moronic. Government and military officials would surely realize, like in OTL, that a Carthaginian peace is good for no one. I think that any "de-Freedomization" campaign would be ended after a few years, like OTL's denazification. Instead, propaganda would emphasize that Yankees and Southerners are the same people, and that the Freedom Party was a minority, who misled the majority. I could see a "Clean Army" myth also develop.
After the population reduction, the South will have a massive labor shortage. The South will desperately need immigrants, and immigrants will surely come. Now, there will be a nativist backlash (as all immigration waves are accompanied by), but, given the economic situation, I doubt it would be very successful. The popularization of air conditioning and cheap living costs will probably also result in Yankee migration south like in OTL. By 2020, I wouldn't be surprised if most of Dixie's population is composed of immigrants and transplanted Yankees and their descendants.
While there were may differences between North and South, there were many more similarities. They speak the same language, follow the same religions, have the same history (until 1861), revere the same Founding Fathers, have the same political values, celebrate the same holidays, etc. After a while, the former Confederate States would be readmitted as states, and it would be difficult to tell that the USA was once two countries. I think even among the descendants of former Confederate whites there would be little interest in secession.
Annexing the Caribbean wouldn't be too big of an issue either, since the those islands have and had small populations. The two largest the US would annex, Cuba and Jamaica, had populations of 4,778,583 (OTL 1943) and 1,403,000 (1950). And, at least in the Bahamas, Bermuda and Cuba, the US would be very popular after the war, since Yankees liberated those islands from Confederate occupation.