Agreed that nobody wins in a trade war, though, the threat of limiting the oil flowing south isn't very credible. Canada is already hemorrhaging money due to the downturn in oil pricing and the subsequent devaluing of the Loonie. Also, Canada doesn't really have any other options of where to send the oil, as thanks to a variety of political actors, Canada hasn't been able to build a pipeline to the west coast to move the oil overseas to places like China. Lastly, I don't even think that the Canadian federal government could reduce the flow of oil, as the sector is in private hands. I suppose something might be able to be done with cross-border rail movement, but I'm not sure. Regardless, it would be cutting the nose to spite the face, so the point is pretty well moot.
As for the ability of Canadians to create their own policies of self-government solely on internal politics without regard for geopolitical implications, I'm not going to pout over it. Geopolitics and international relations effect the internal politics of every country, and American concerns are not without merit.
If a Democrat wins the WH, I'd be surprised if that threat were levied, as it would anger much of the Democratic base. However, who knows what the future will hold. Law enforcement agencies certainly hold a great deal of policy sway in the US (as in most countries), and my take is that they're more anti-legalization than the average American voter and politician.