Well you see this kind of theory brings about a problematic that I think is largely absent in the context of scandinavia and acts as a rather large elephant in the room for these kind of discussions - and that is it suggests the adoption by non-natives of beliefs systems and folkways that have been aggressively and ruthlessly repressed both by Christianity and it's various cults, and the major states involved (US, Canada) on economic, cultural, and racial lines. These two states, and the cultures within them, have also explicitly constructed themselves opposite to these native cultures. There is no return to "indigenous" or "pagan" faiths in America because a lot of the nations of America were founded as explicitly christian European nations. You can also imagine that natives would not be keen in welcoming in welcoming for a second time plague-bearing foreigners to their shores. So we can see a two-fold resistance - from within the american cultures themselves, which have some 200+ years of narrative about the inferiority of native cultures, and from the native cultures themselves, who I doubt would be very keen seeing the last little bits of land they have be overwhelmed once again by americans and canadians who also now lay claim to their culture as well.
So would natives persist - considering the amount of crap they went through already and their current geopolitical situation (herded in isolated reserves or territories, mostly, with very little economic options) I would say yes. Would they revert to their old ways - arguably yes. Or at least, as much of the old ways that are remembered or re-imagined. This has some leeway because it is not clear how otherwise overwhelmingly Christian nations reverted to paganism in the space of three generations - I chalk it up to the fact that the kind of apocalyptic plague going on is enough of a disruptor to bring about massive social changes in a very short amount of time. Of what I know of aboriginal folklore there is plenty in there to keep them safe from trolls. The major issue would not in fact be trolls, but rather beasts. Since northern Canada (and US) is aggressively hostile to most form of agriculture, hunting-gathering is the de facto answer - a practice that is directly endangered by the plague spreading to mammals. So it would be an even more fragile existence than what have seen so far in the comic. A herd gets decimated and a whole nation may starve during the winter.
So really, it's not the plague that is the real problem - it's the winter. It's those 6-8 months of crushing cold. But they lived through it in the past, so I am sure they can do it in the post-apocalyptic future as well.
I am confident that the Canadian shield rock formation and terrain, as well as the valleys of the Rocky mountains, are more than enough deterrent for monsters, but the problem is to eke out a living out of such places. Which is virtually impossible beyond a certain population mass - made even more difficult if the primary sources of food are directly affected.
So really - if there was an exodus to northern Canada the primary result would be widespread famine and death and hostile populations and death. The populations already there would have to adjust to losing the infrastructure (money, food, materials) flown in from the south, but eventually it is in the best place to build a post-plague society. The problem is that in North America there is nowhere that can play the role of Iceland as a stable source of resources and foodstuff.
We are talking perhaps 30 000 - 80 000 at the time the comic takes place, I would say. 80 000 is generous and presumes surviving cultures in the Saguenay, Newfoundland, some of the islands of the Pacific Northwest, as well throughout Northern Quebec + Northern Ontario + Northern Manitoba and Saskatchewan (Crees), the Northwest Territories (Dene and a bunch of other groups, Inuit, and Nunavut + Nunavik (Inuit). I also include French Canadians, Acadians and Metis as possible culture because of all the european-descended nations in America, they are as far as I can tell the only nations that had any meaningful cultural exchange with their allied native nations and with a foot in the door to adopt these beliefs. But that side of the culture has also been suppressed and was largely tied to the fur trade. This is important because in this world rivers would once again be the veins of commerce and transport. The huge network of rivers and lakes of the north would make it possible that these 30 - 80 000 thousand people survive but also stay in relative contact. A very loose confederation of outposts, cities, and campgrounds with possibly more than 80 percent of the population living a non-sedentary or semi-sedentary lifestyle of hunting-gathering, fishing, voyaging, foraging. Probably very different lifestyles depending on the season, too.
Also Mikmaqs are too much to the south to be good candidates for a surviving culture. New Brunswick is not great but it's not that barren either. The would be overwhelmed unless they moved further north and other side of the Saint-Laurent.
So with these reservations I think we can direct a better line of inquiry and worldbuilding.