The problem here is that the basic founding premise is fatally flawed. The United States wasn't going to tuck it's tail between its legs and go yipping home if Overlord failed totally. Ike would have been axed (probably without good reason) but the United States wasn't going anywhere.
The U.S. didn't fold and leave the table after Pearl Harbor, after losing SEVENTY THOUSAND men in the Philippines, after Guam, Wake, Kasserine Pass, Anzio or any of the other disasters during the war.
The likelyhood of the entire landing failing is also vanishingly small. The Allies had total control of the air, to a degree that was (and still is) stunning. On D-Day the Luftwaffe managed to have 300 or so sorties against the beachhead (as a comparitive, the Allies put 171 SQUADRONS of fighters up that day), the Allies had almost 15,000 COMBAT sorties on 6/6/44. That sort of difference is unrecoverable.
While the popular vision of D-Day is that the Germans almost managed to defeat the landings by holding Omaha, that is all it is, a popular vision, one that movies like to play up. It is also a misconception. Had the Germans managed to totally destroy the Omaha landing force, that would have cost the U.S. Army TWO divisions, out of ONE HUNDRED, (in any case, Omaha was secured by 14:00 on D-Day). The absolute "best" possible result the Germans could hope for was an "Anzio light" where the Gold/Juno/Sword and Utah beachheads are contained. Even that is unlikely, again because of the airpower available to the Allies and because of the HUGE amount of manpower the Allies had in England waiting to reinforce the landing forces. Unfortunately for the Germans, unlike Anzio, the terrain surround the beaches in France wasn't comprised of a series of ridges that provided the Heer with perfect locations to site artillery, nor was it at the edge of range for Allied fighters. Allied aircraft had substantial loiter time over the beaches thanks to the proximity of their operating bases and the RAF Bomber Command and 8th Air Force had well over 2,000 bomber within 300 miles of Utah, the most distant of the beachheads from the bomber bases. At best, the Germans would have been forced to concentrate their forces in the open to contain the beachheads subjecting them to murderous bombing and strafing with inevitible results.
Normandy wasn't the only place to attack France, as the allies demonstrated diring Operation Dragon. Normandy was a very good place, because the Germans didn't expect the attack to fall there, but even after a repulse there the Allies had the forces and logistical ability to go again. This also mitigages against the forst part of the initial scenario.
However, let us leave those three significant items aside and assume that the Germans hold France until the Soviets are entering East Prussia in force (February of 1942). At that point, even Hitler will pull forces out of France to stop the Red Army. Guess what happends then? The Million or so Allied troops in the UK roll over the dimished German Defenses and make it to the Rhine in around three months. In this scenario, the Allies meet the Red Army at the Rhine instead of the Elbe. This will have huge butterflies, but nowhere near the size or number needed to make the scenario fly.
Now, lets go a step further and imagine that, against all logic and pre-war planning and all agreements made during the war, the allies don't make a second attempt at France. Vanishingly likely, but WTH?
No second invasion instead frees up additional forces for use in the Pacific, not that any were needed IOTL. The U.S. takes out its frustration from the ETO on the Japanese, crushing them like an empty beer can (wait, they did that anyway... oh well) The Americans come home with Japan's scalp and count coup, having nuked the Japanese (and, depending on how the war in the ETO went, maybe a couple German cities as well) and obtaining unconditional surrender. The U.S. military comes back from the war, the most powerful military force on Earth with a massive victory to take the sting out of Normandy. It is now confronted by the USSR which is feeling it oats, so the U.S... Disarms? Hardly.
In this scenario the U.S. puts even more emphasis on Naval and Air Forces, perhaps not even demobilizing the Mighty 8th since they are facing the much stronger Red Army across the Channel, which is the front line in America's Atlantic Wall. So the U.S. has MORE bombers, a BIGGER

navy, and is even more paraniod about Ivan. Not going to get an invasion of the U.S. under those Conditions either.
Hmm..., How about if we go WAY into the unlikely part of the pool? Japan WINS in the Pacific (yes, I know I've got two dozen recent posts demonstrating why this is impossible, still...)
Now the U.S. has enemies on both sides, who have demonstrated that they are expansionist and angry at the U.S. So now, as a result, America... disarms. Yea. Not gonna happen.
Now you have an America that does a reasonable imitation of Switzerland or Israel, except with a Thousand ship navy and Ford, GM & Chrysler still building three tanks or planes for every car or truck. Turning itself (and probably Canada, who will be as freaked out as the U.S. in this scenario) into a hedgehog with a clearly stated First Use policy and a paranoia about Communists and Orientals (they're all the same, don'tcha know) that knows no bounds. The best literary example of this version of America? The Terran Federation from
Starship Troopers where the military is a central part of everyday life. Now, the USSR decides to attack this bunch of heavily armed paranoids? Always knew those Commies were insane.
So, exactly where, in any of these scenarios, does the U.S. return to 1927 attitudes? Exactly nowhere.
One last point:
I just wanted a Canada that didn't have it's armed, air and naval forces cut to bit after WW2 due to the Americans
The United Stated didn't force, or even ask, Canada to disarm. Canada is, to be brutally honest, along with the Atlantic and the Pacific America cushion against attack. Canada is also American largest trading partner and a steady respected friend and ally (longest unfortified border on Earth, remember?). The best thing for America is a STRONG Canada.
Canada disarmed because CANADIANS wanted to disarm. They elected politicians who made that a priority, who made a series of decisions that created current Canada with the social benefits it affords to residents. Canada's population enjoys a number of Government provided programs, perhaps most famously Universal Health Care, that do not exist in the U.S. which absorbs, by itself, 7% of Canada's budget, with social support of other sorts equaling another 7%.
The U.S. put almost 25% of its total budget into defense. This from an economy that draws from a population of 300 million, while Canada draws from 11% of that number. As near as I can figure, the U.S spends as much in CURRENT military spending, not including the Iraq warm as the TOTAL Canadian government budget for 2005-2008.
Weapons are expensive. Not every country is willing, or able to spend enough to support a major force.