I think you're misguided in trying to map developments in Nazi Germany to today's North America, just as Atwood was in trying to apply revolutionary Islamist Iran within a Christian context. Not only do people know how the Nazis' story ended, but Germany was always a vastly more authoritarian construct compared to America or Canada, as can be expected from a country united by the famously martial (bringing with it an extreme emphasis on things like discipline and regimentation) Prussians. In terms of religion it already had a strong trend of state meddling in theology, what with all the established state churches in Germany's constituent kingdoms during imperial times which survived into the Weimar Republic and then, of course, Nazi times, and Bismarck's attempted
Kulturkampf to crush German Catholicism.
State churches however are absolutely not a thing in the US, and haven't been since the 1830s when the last of them were disestablished. In Canada you have the Anglican Church I guess, but they're in steep decline like all the other mainstream churches which have embraced worldly positions and nobody cares what they say, not even the Anglicans themselves. Thus, there's nothing there that can serve as the backbone to any great American equivalent to the Deutsche Christen. American and Canadian Protestants are a famously fractious bunch: extreme racist and extremist types might be able to take over one church here and there, but there's no central organization in the vein of the German Evangelical Church Confederation/German Evangelical Church that they can hijack to force every other church in the land, or even most churches, to play by their rules. You can't just ignore the specific peculiarities of a nation's history and how that historical context will inform trends when predicting these nations' future course, especially as Christian reactionaries are more likely than most (much like any other stripe of religious reactionary) to look backward to find their way forward.
Keep in mind also that the Deutsche Christen/Positive Christianity types actually
failed in their drive to take over German Christianity (as evidenced by the Confessing Church's existence and most German Christians not caring for either camp). Ultimately the Deutsche Christen and Positive Christianity were both primarily pushed by pagans and occultists whose primary obsession was always racial purity and Nordic pagan woo rather than the Gospel, which was understood and held in contempt by actual practicing Christians: you can compare that to alt-right 'leaders' like Richard Spencer and Nick Fuentes today, who have already become laughingstocks mocked and despised as, respectively, a 'beefsteak' wannabe-tyrant & probable fed and an absolute degenerate everywhere. The IRA was also more nationalist (and even socialist) than it was Catholic, Catholicism being treated more-so as an extension of the Irish national identity rather than being treated as their overriding higher cause as it was in the Crusades (note that the nationalist parties claiming IRA heritage tend to be the most left-wing today).
Worst comes to worst, any ultra-violent and extreme future Christian movement in North America will at worst resemble the KKK (of which, to even tangentially tie this tangent to Canada, I must say there did exist
a Canadian branch back in the day), not the Nazis. And even then you'd probably need, like, child-raping orgies at altars to piss North American Christians off badly enough to go that far. Demographics - what with Protestants not significantly outnumbering Catholics either in America or Canada, and Catholics being the largest single denomination but still outnumbered by the Protestants when the latter are combined - will certainly make it impossible for any single sect to assume unitary leadership and impose its dogma on the others: if they persecute anyone it'll be the liberal-progressive churches, like the United Church and Anglicans here (assuming those still exist when a proper reaction gets going, which isn't all that likely considering their demographic trends), but the instant they try to impose any order among themselves beyond at best a Nicene Creed-esque understanding of the most basic tenets of Christianity (like 'the trinity' levels of basic - Unitarians who reject that have given up being Christian at all, see modern Unitarian Universalists) any unity will break down quickly, making it inadvisable and unlikely as long as the threat of Globohomo exists. (If a pan-Christian reaction implodes and is overrun by Globohomo in turn then there won't be any Christians to turn 'ISIS' anyway.)
As to what might happen if a latter-day KKK-esque movement should decide the time has come to purge the others when the great rainbow beast of modernity lies dead, refer to our past discussions in PMs about 'fundamentalists' vs. 'ecumenists'. Speaking of which it would absolutely be for the best if you move this discussion to PMs, if you're interested still in carrying it out, because I've just had to go to the length of digging up the Canadian KKK to keep this tangent even remotely linked to the actual thread topic of Canada and horrors associated therewith.