To put my two cents in. Having tried and ultimately failed (I should put emphasis on failed) two TLs in which War Plan Red is activated I have to say that it is highly unlikely the plan could have ever been sprung into action.
Though from a purely speculative POV I'd say the US would have trouble sustaining an invasion on a few grounds.
1st is that in any scenario taking Toronto and the Windsor corridor right off the bat would be tricky. Enough built up areas to grind down any advancing Blitzkrieg of tanks and the corridor itself is quite small so that would be the most hard fought area.
2nd is that neither the US or Canada had the forces on the West Coast to really make a battle out of taking Vancouver and if the Americans failed to take it Australian troops would just pour in making it harder for the US.
3rd British Empire would definitely intervene on Canada's side. The threat to the Caribbean and Latin American trade routes would be too great.
4th is definitely guerrilla warfare. In the Windsor corridor it would be more difficult (bigger concentration of US troops) but on the plains and in the Maritmes Canadian Partisans would make American soldiers lives hell. On the plains sure the Americans would rule in the cities but the rebels would stick to farms and woods. What could the US do? Bomb every cow into oblivion? In the Maritimes more access to British guns so better resistance.
Rebels might not win but they could make life hell.
1) Yes
2) No. Unless you are suggesting the Royal Navy can defeat the US Pacific Fleet in the Eastern Pacific within land-based US air AND with the Australiams traveling from one end of the Pacific (SW) to the other (NE), more than half of which is through hostile waters.
3) No. Land-based air again. At least in the Northeast Atlantic, Gulf of Mexico, and much of the Caribbean.
4) Yes, but without support from the UK major league resistance (except in the Maritimes) will be difficult to do for the Canadians, and could get very bloody for both sides.
As others have noted, in a period of increasing tensions, neither side would have the anemic forces that existed IOTL. As to Britian's greater naval strength? By the 1930's, that was mainly only in ASW, not a serious threat in terms of naval battles around North America. The US had a slight advantage in battleships (though much slower), aircraft carriers (aircraft numbers and quality, carriers quality), and heavy cruisers. Equal in destroyers and submarines (though better quality for many British subs and their torpedoes). Huge advantage for the British in light cruisers, even more so in destroyer escorts (which the USN had virtually none at the time).
And just as the USN's commitments in the Pacific (facing Japan) force a large proportion to be left there, it also makes a British campaign against the US in the Pacific/US West Coast a dead letter proposition. And British Imperial commitments in the rest of the world will not vanish overnight either.