Such US means would be worthless against the UK. With its Navy, the UK could completely asphyxiate the USA of the 19th century as it could do with any other country. Bomb and destroy to the ground all its coastal cities.
In such a war the USA would beg for peace, restitute any british territory taken. And you may even imagine some coastal States seceding.
To prevent this, the US would need a very big allied coalition, like for the ARW. But there was no such possibility after the defeat of napoleonic France.
Basically I'm challenging the notion that there must be some sort of huge showdown over slavery with such an early and massive change to OTL America.
Since New England was de facto neutral, why not make Canada slave states for the heck of it? That or an agreement that Canada would be free states while the land west of the Mississippi would be open for slavery?
Canada was a popular destination for escaped slaves. Take away Canada, and that does bring up an interesting question: where would the terminals for the Underground Railroad be? Newfoundland? Surely not Greenland. I suppose they could try for Florida and become some early version of Boat People, trying to sail to freedom in the Bahamas.
To forestall any Civil War, slavery might be extended further into the Midwest for that pointless balance in the Senate to continue. Nebraska and Kansas slave States? I don't think cotton would grow well or long out there.
Britain would not support the Monroe Doctrine the Americans would need to enforce this on their own (meaning more resources in their navy)
The UK might not be in a forgiving mood, but in 1814, she was clearly exhausted. If Quebec had fallen though, the British might have gone for revenge by starting colonies along the North American Pacific coast. And there would have been little the US could have done about that.
It was a sideshow until Britain is free to do something about it. Once they are they take it back. If they are not able to take it back it's because they are not in a position to do so, ie the French have won. What peace? The US cannot make them come to the table if they choose not to, and there is nothing forcing them to. "Oh no! Canada is occupied, well i guess we will just quit then." Very good quitters those British
The only way Britain is beat down enough to make peace with the US on anything other than amicable terms is a Britain that is wrecked by Napoleon.
On what are people living on that they think a Britain more battered by Napoleon, or decisively defeated somehow in North America is going to commit to the expense of liberating colonies so sparsely settled and of little economic value at the time?
On the question of slavery, while the abolitionist minority wasn't as yet a significant political force in 1814, the pro-slavery South also hadn't quite consolidated around the idea of slavery being a positive good in 1814 either. Either way, states rights were still paramount, and despite the earlier fugitive slave act, it was still mostly a regional issue.
I actually don't see slavery being much of an issue in this particular scenario, but I do see it inevitably leading to a weaker South once King Cotton changes southern attitudes in a decade or two. This might lead to either a humbled south (less likely) or a South more insistant upon greater annexations of Mexico, perhaps even to the point where the entire country is annexed in the furtue, and indian slaves become as important in certain areas of a truly north Americans "Union" as black slaves in the old south.