Why in the world would anyone consider the United States to be a Great Power at this time?
Well, Victoria 2 certainly considers them a Great Power at the time.
In all seriousness, there's a difference between just being a Great Power and being near the top of the list. The US had already managed to project its influence worldwide, as can be seen by the multiple foreign expeditions to the Barbary States in decades prior, the establishment of a treaty system in China along similar lines to the Anglo-French treaties established after the First Opium War, and so forth. So, while certainly not the greatest of powers, they did have quite a bit of influence worldwide at the time. I can't give you a quote either, but I'm rather certain that by the 1830s they had one of the largest merchant marines, not matching the UK but being close. They did not maintain a large army, but that was mainly a result of their geopolitical position which meant they didn't need one.
Though I fear this just leads into a debate on how to define what a Great Power is.
Not possible. The Upper Canada "Rebellion" was actually a riot, of which there were many in the Canadas in this period. It was really the result of the Chartist-like early demands of the Upper Canada reformers not being met that it eventually radicalized, and ultimately some of their demands were met after the failure of the rebellion. The idea of Mackenzie's earlier demands being met is a far more plausible one.
Roughly my impression, though I wouldn't quite call it a riot myself, but it was very definitely a fairly minor event all around. As such, I wouldn't expect the rebellion as it happened OTL to have any chance at all of succeeding.
I would never say that some alternate rebellion couldn't succeed, but it'd take some rather major PoDs. Perhaps the initial land grants in Upper Canada were delayed, meaning that the province remains relatively undersettled. Eventually, in order to fortify the frontier in the face of American settlement elsewhere, they must start issuing land grants. And, as in OTL, the settlers coming across would be majority American. But, as it is later in time, the settlers, having lived in America longer, are more detached from the former monarchical government and, in the end, more pro-republican than OTL. (though, this possibly diverts the War of 1812, perhaps averting it altogether).
Then you'd have to have greater resistance internally for reform movements to have a chance to spread. an alt-Chartist movement could be more heavily repressed. Without the greater sense of Loyalist Canadian identity brought about by the War of 1812, a greater internal republican conscious, and combined with greater repression of the reform movement, and you could see a full-blown rebellion breaking out.
Combine that with a US that was never damaged so heavily in the War of 1812... Well, you might get more active support.
Though, I would find that this all happening very.... unlikely. And it would affect the rest of the timeline drastically.