<<If Canada really expects US help when fighting, should the US expect Canadian help when fighting?>>
In the real world the likeliest country in the world to invade Canada is the U.S.A. I would not expect the U.S.A. to help us in repelling an invasion from the U.S.A. if and when their need for resources becomes so desperate that invasion looks like a good option. We have no effective means of resistance. We are not Taliban.
I would think that before any invasion could occur, sea-changes would have to appear in the American political landscape that would be obvious to most observers. At that point it would be in our best interests to look for good defensive military alliances. China would be one such. We need to align with winners, not with losers. Right now the U.S.A. under present management is a loser nation. Losers are dangerous precisely because they feel impelled to prove they are still winners, and the easiest way to do so is to attack a weaker victim and humiliate it militarily. Canada would be an excellent victim. If the present disastrous course of the U.S. continues unchecked, its long, slow slide into militarism and fascism will continue at an ever-accelerating rate.