and subject to a Charter from HMG. HBC territory was for them to use but not to give away or sell.
This.
The US is NOT going to 'buy' the HBC and gain the territory.
What COULD happen is that American colonists flood into what is now the Canadian prairies, and they end up petitioning to join the US. Basically what happened with the southern chunk of Oregon territory.
The Brits/Canadians are not going to hold down by force an area that has a sizable (white) American population.
Only after the US controls the habitable parts (the prairies) would discussion be possible about the HBC and land north of the tree line (i.e. where trees start, not where they end).
Even then, it might well be possible for Britain to keep most of the Canadian Shield, for instance, if they wanted to. Not sure they would, but the US might not want that territory much either.
As for a PoD, the TransCanada Railway was incredibly difficult and incredibly expensive (for a colony the size of Canada - even the expanded Canada after 1867). The project could have failed several times, politically or economically (realistically, a mixture of both).
Without the railway, Canada / Britain has no way to project power / military force onto the prairies in the winter, and its difficult even in the summer. IIRC, during the First Riel Rebellion (the one that ended with Manitoba as a province), the force had to transit THROUGH the US. The Second Riel Rebellion, there was enough of the RR built that troops (sorry 'police') could ride the rails where they existed, march to the next stretch, take rails again, etc. And the whole path was cut clear to allow RR building.
If the RR is never built, the West will fill up with US settlers coming north from Minnesota, down the Red River. And all produce will have go south up the Red River to markets. In a decade or two, this alt West will be so dominated by the US (economically, if no other way) that the only logical end would be the prairies joining the US.