A decisive defeat for the US likely means a higher number of casualties. That's going to impact local growth. Immigration will be impacted as well. Also, a civil war is not as implausible as you may think. As I mentioned, backlash over the war will at least cause lots of friction between sides. Opposition to the war was considered as treason by some. And with stronger voices of opposition, what do you think will happen then?
It can work since the US isn't likely to do much against them. Yes, perhaps at some point it might integrate into Canada once the US recovers and becomes strong enough to now challenge British North America (or the US conquers them, whatever happens)... but how long do you think that may take to happen? Consider then the buffer state might becomes the new destination for tribes opposing the US. Suppose the Civilized Tribes are still expelled from the southeast. Do you think they wouldn't flee instead to the buffer state... or ally with them if they still move west?
No it's not going to impact local growth, it's just not physically possible to kill that many Americans with the type of warfare in this era.
A civil war is not implausible, what's implausible is one so strong to stop a trend going for 3 decades by now.
I don't know how to phrase, but you can't remove the growth just because, there are reasons for it existing and simply having civil wars isn't going to change that more so when the scenario present isn't that plausible to begin with, yes friction would exists, New England could declare independence and be re-annexed forcefully etc etc. this isn't going to kill hundreds of thousands of people or decrease fertilty rate, this is not a world war scenario.
It isn't about the US, but about the functionality of such a state and its usefulness for the Brits, the Brits themselves would soon find the land the states encompasses to be good land for further settlement especially with the USA expanding demographically in the region as well and 50k people forcefully removed from the Souther United States isn't going to fix that either, how much population would this area have anyway?
There were hundreds of thousands of Brits and thousands of Canadians who ended up in the United States
What do you mean by that exactly? US didn't experience much migration until 1830.
A HUGE percentage of America's growth was through immigration, by 1840 roughly 40% of the major port cites (New York City, New Orleans, etc) were foreign born.
Sure, although by then foreign born population was still less than 10%, less than Modern USA anyway. You aren't going to shift that many people and even if you shift all post 1820-1840 immigration you aren't going to lower US population by more than 1 million by 1840 and that's the worse case scenario.
The territory in question wasn't even settled in the time period in question.
No it wasn't but it would soon be, by 1840 it was already fully settled.
By the time that Americans trickle north from the Ohio River into northern Indiana and Illinois the Canadians will have been trickling south from Michigan and the lakes. Americans will be flooding the same areas they went to OTL first. The northward migration only happened after all the better land further south. The Canadians will use the St. Lawrence and Great Lakes as a highway and go west. They'll have the easier time than the Americans.
Michigan ok, but I'm not sure how they are going to settle Illinois or Indiana not in any large amount, the US by 1840 had already more than 1 million people in both states, around the population of all the Canadian colonies around the same time, the Canadians would have to fuel most of the settlements with migration from Europe and that's not possible at such speed.
IMO there is no reason the Brits would try to keep a undefensible border by annexing land south of Michigan for no reason other than piss the americans off and waste their money and resource for something they OTL though of giving off to natives to maintain the fur trade, Canada isn't going to be able to just replace US expansion by just somehow taking the few migrants US had until 1840, when they already fully settled both Illinois and Indiana.
Best Canada
Disclaimer: I did not make this map
Not sure if that's what others suggested as British landgrabs, but I don't get why the UK would be ok with taking 40 km wide strip of land to cut of Illinois from the lake Michigan but allow the US access to the pacific, it's illogical.