For that 40/60 do you mean that it would be a 40% change of the British not trying to keep it or a 40% of the Britain not managing to keep it at all? Because IMO I have hard time believing that if it comes to war that the US wouldn't seize relatively easily, especially by the time railroads reached southern Illinois. Would the British really even want it to come to war against the US?
Also about the 4-5 million figure, why would Canada get so much many more people just because of Michigan and Northern Illinois? I mean I get them having more land directly free for settlement but Michigan in 1850 had just 400k people and altogether I don't imagine Canada going over 3 or 4 million people even with immigration mostly because from what I see most of US growth was actually local growth until the 1840-1850 period. As I understand Canada had about 2 million people in 1850 IOTL, with Michigan, Wisconsin and Minnesota I can see them reaching 3 million but in itself would those annexed areas really have much more people than they had under the US?
40% Britain keeping it and 60% selling it off later.
As to another war, it would depend on a) The Americans feeling up to challenging the British again (not incredibly likely within 20 years of peace IMO) or b) the British feeling that the investments made in the region are worth defending. Historically the defended Canada on principle vs. for its greater economic benefits, but if Canada becomes a more profitable colony then they might see it in their interests to press the issue. And when they were not exactly winning a definite victory they were more cautious, but in a scenario where they win decisively, well who can say?
4-5 million includes immigration and natural increase. Higher immigration probably tips Canada over the 3 million mark by 1850, but with larger settler families (and period growth rates) I'm willing to bet the population is about double its historic size by 1850, and by later in the 1800s is probably pushing 5 million. This does assume higher settler rates, largely on the basis of canal projects attracting workers and more land being opened for settlement "filling" Ontario sooner than OTL which pushes casual settlers west while increasing cities like Kingston, Toronto, Hamilton, Montreal, Quebec, and Detroit.
Admittedly 5 million by 1850 is a bit of a stretch, but 4 million is easily doable with the land Canada has to offer even before including any land seized in the war, here I'm just positing bigger growth rates from a healthier economy and higher settlement patterns.