I've been reading replies, taking notes, and preparing this response for days, so be prepared for a long one.
Firstly, comparisons can be unfair. Modern Japan for example is not the same as feudal Japan and will (like Iceland) struggle to feed its population, so that historical comparison can only go so far. Assuming population drops will be equal across the board isn't really fair either, not every place would face the same exact challenges as Iceland, Denmark, and Fenno-Scandia or face them to the same extent.
Steingímur Þórðarsen (the in universe creator of the map of the known world) is not omniscient so some details may not be absolutely set in stone, but some interesting details that some people may not have noticed are that some islands in the Shetlands and an isolated island in the Baltic are marked as cleared on the map. Someone who didn't notice this asked how it would be possible that Iceland would contact Norway before the Shetlands, to which Minna replied that the population of the Shetlands and Outer Hebrides could have been assimilated into the Nordic countries. Some people seem to take the possibilities Minna mentioned as definitive proof that the people of the British Isles have been entirely infected or assimilated but I'd like to bring to light the heresy unorthodox interpretation that Minna was just exploring potential explanations and there could still be Celtic or Anglophone survivors out there in the fanon or canon. The Baltic island is something of an anomaly, as (IIRC) it is uninhabited, however it might be connected to the small Eesti speaking population shown on the language tree psge.
We don't know much about how patient zero contracted the rash, where they came from, and most importantly whether the people back there were already dealing with the rash or a related strain of it. If there's another place besides Spain that the rash is spreading from or another disease closely related to the Rash, it could be very important information on what places could survive to year 90. (Unimportant sidenote: Even if someone else had contracted the rash earlier, patient zero would still be the first patient diagnosed with the disease)
Demographic changes (like the surge of neopaganism in the Nordic countries) are to be expected in other places for both Doylist and Watsonian reasons.
Communal support, culture, religion, magic, the gods' protection (a phrase including, though not necessarily limited to, immunity) are important to survival in the SSSS world, but we can't really predict how they would change, and to say a population homogeneously fails in each of these respects and couldn't change even if it were life and death would probably be more than a little bit offensive.
Thematic/narrative importance trumps all else. Expect any existing survivor groups to follow the established themes and trends and not disrupt the neoviking narrative or linguistics exploration too much. That means don't expect expansive military, cultural, or economic powers or alliance to overshadow the known world.
Mutual support with nearby survivor groups, as the Nordic countries give to each other, may not be strictly necessary to surviving to year 90, but they sure would be helpful.
Isolation, however is also helpful, as it may allow a location to avoid the rash in its entirety (as Iceland did) or atleast buy time to respond to it. Japan, Madagascar, and Iceland were known to have closed their borders early, before the news of how bad the disease actually was had even broke. Were their respective leaders privy to more information than the news let out at that time?
Although doing a page 51 did prevent Iceland from ever getting the rash, we now know that the ideal procedure would be to have protective equipment and quarantines as Iceland does by year 90, however early on it may be impractical to supply everyone with proper equipment and to quarantine all refugees. Existing public health, sanitation, CBRN, civil defense, and similar resources would be very helpful.
A central government that is authoritarian enough or regional governments that are independent enough to enforce quarantines and control movements within the country would be beneficial in preventing the disease from spreading throughout the country.
Topography, ecology, and climate will play important roles in resistance against trolls and beasts once the rash has spread through a country. Topography will be important for defending and reclaiming territory. Climate will be important as trolls prefer dark, warm and damp places, although beasts are more resistant to the climate than trolls are. The ecology is important, as some animals may serve as vectors for the disease (most notably mammals, though ticks, fleas, and mosquitoes could also carry infected fluids, even if they're immune) while other animals (most notably felids) can help eliminate these vectors. Plants and animals can also provide transportation and important resources for food, shelter, fuel, etc. By year 90, atleast, ecosystems have recovered enough to amply supply what small pockets of civilization may be left. Other important resources a location might have for survivors include drinking water, shelters, armaments, transportation, minerals, fuels, etc.
Population density is another key factor, as it can generally be assumed that lower population densities won't spread the rash ad quickly, however it should be noted that statistics on population density are very easily skewed when looking at large areas including major cities or by including or excluding water from the calculations of the area. It's also worth considering that if the population enters a panic, they may emigrate out of large population centers, possibly bringing the rash with them.
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