
So yeah, DOW Canada and take Frederickton. The British will surely spend every available pp on ground units to deploy near Ottawa (at least I would if I were PanzerGeneral). One interesting question this brings up is where do the British convoys go once England and Halifax have fallen? Will they just stop spawning altogether? Knowing the answer to that question might determine what kind of resistance you face around Ottawa. Maybe you should playtest or something, because I honestly have no idea how that would work.
The only other downside is that now PanzerGeneral knows the Americans have to be ready for a war on the ground, so he will probably start recruting infantry units (at least I would if I were him). But even if you don't take Ottawa, the Western Allies will spend all of 1942 and possibly longer just forcing you out of North America. They won't even get to England until 1943, which should give you plenty of time to crush the Russians. I think capturing all the capitals is well within your grasp.