I think a giant, "uber" Canada is unlikely. Heck, the Canadas (the eastern half) and BC only got merged in the first place (buy acquiring the HBC held land between) to make a solid bulwark against possible U.S. takeover of some of that middle area. Here, the U.S. is in no position to take over anything, so BC & the Canadas are likely to remain separate colonies and - later - separate countries.
Further, during the Napoleonic Wars, Britain was nominally allied to Spain, who had originally "owned" the Louisiana territory. As such, I'd suspect that Britain, not having any great need for more miles and miles of miles and miles, would return those lands to Spain, at least initially. They'd probably ensure that the northern border was set at the HBC's southern border ("all lands that drained into Hudson's Bay"), but that's about it.
However, come Mexico breaking away, this territory becomes...negotiable. Mexico is unlikely to be able to control it (heck, it couldn't even get enough settlers for Texas, thus the mess that was inviting in some American settlers), but at the same time, Spain isn't going to be able to control it either, given it's just a big empty place, with New Orleans its only real European city.
Thus it's likely that people from the Canadas and even more so from the "United" States will start slipping in and making their own colonies. Some might end up states allying with the U.S., some might join it (there being not all that much difference between the two), and some might just end up fighting it over territory.
Heck, the states east of the Mississippi might well end up fighting each other over territory. Certainly they did so during the Confederation days that were - and it'd be unlikely to stop during this extended Articles period.