British Federation

Absolutely.

War Plan Red

In PLAN RED, the Atlantic Strategic War Plan, the strategists theorized that there would be a war with Great Britain. They did this because England was locked in a strategic alliance with Japan, the Anglo-Japanese Alliance of 1902, which was renewed and lasted until the Washington Conference of 1921-22. American planners thought that England’s imperial reach would bring it into conflict with the US.

Another contingency war plan they developed was the RED-ORANGE PLAN, which hypothesized a two-theater war, seeking to win first in the Atlantic, against England, while fighting a holding battle in the Pacific, and then defeating Japan. When World War Two broke out, military and naval planners simply dusted off the old RED-ORANGE PLAN and substituted Germany for England in the Atlantic Theater.

http://www.globalsecurity.org/military/ops/war-plan-red.htm

War Plan Red was drawn up in the 1920s. Violation of the Monroe Doctrine would absolutely, certainly bring it into effect, former ally or not. The US has had a long habit of turning on former allies. Japan was an American ally in WW1, for instance.

And, do you REALLY think the British public would support a war or police action against Canada, without even a cassus belli, right after WW1? 

Like I say, if you want to have a British Federation of some sort - or at least a Commonwealth that remains a superpower - it could theoretically be done, but not by the methods you're talking about. A more likely scenario is that Britain does not go to war with Germany during WW2 and never accedes to Bretton Woods or the Atlantic Charter - this would probably mean that the US never quite succeeds in becoming a hegemon, and the Commonwealth remains a superpower. 

You asked what you need to make this plausible. I'm telling you that anyone with knowledge of Commonwealth history will not find this plausible at all, the way you have set it up. Britain is in no position to exert additional control over its colonies in the 1920s, and its not in Britain's interests to do so at that point. There is a very long-term trend at work here, which began with the factors that led to the policy of "responsible government" (google it) in the colonies. They were ungovernable from London without gradually letting out a bit more slack on the leash over time. A kind word and a gun wouldn't stop this even a little bit, they tried that and it failed spectacularly everywhere they attempted it (India, USA, etc). To stop the necessary process of devolving power to the colonies, would have been to lose the colonies even more rapidly. Attempting to rule them by force would have eventually failed - yes, perhaps they could have done the impossible for six years like your example of Germany, but at what cost and to what conceivable benefit? What do they gain? They already have the allegiance of the colonies and their support in most British foreign policy initiatives, including war, so why would they suddenly attack their own best allies? 

And yes, the US would absolutely respond to any armed intervention in Canada. Consent would easily be manufactured by trotting out perfidious Albion, the nasty redcoat, "The British are coming!", and so on. War Plan Red was a very remote possibility in the 1930s and 1950s, but in the 1920s - when it was drawn up - it was based on much more likely potentialities. Chiefly, it was about China, where the US had certain commerce interests. It was principally meant to go into effect should the "Open Door" policy be violated by Britain and Japan. How much more they would respond if the Monroe Doctrine, so much nearer to home and so much more important to American interests, were violated!

To create an alternate history where the Commonwealth remains a relevant power bloc, the way I see it, there are only two plausible options. One, is that instead of adopting "responsible government" in the 1840s and 1850s, Britain creates a federated union and brings the colonies into the British Parliament, with MPs and all. How doable this might be is dubious, as it would meet heavy resistance in the UK itself, and its uncertain how much this would assuage the colonies (Commonwealth citizens can, already, vote or run in UK elections but few of them know or care). 

The other option is not a Federation, but just the old Commonwealth as it existed in the 20s and 30s surviving the war as a major bloc to be reckoned with. This is, I think, quite plausible - but only if Britain does not go to war with Germany in WW2, and ensures that the Sterling Area is preserved by refusing to sign on to Bretton Woods or the Atlantic Charter.