The hardihood of rash-infected creatures is definitely a factor, but I think it's eclipsed by an even greater factor.
Namely, the virulence of the disease itself in its initial phase.
I want to point out the difference between Iceland and the rest of (known) Europe in this regard. The total die-off rate from all factors for non-Iceland Europe was something like 99.9%. It was basically TPK.
Meanwhile the total die-off rate for Iceland was under 50%. Assuming that a large portion of the population squeeze was food-related (either immediate starvation or a more gradual controlled population decline over several generations between Y0 and Y90), that means that Iceland was essentially unaffected by the disease.
I want to reiterate that. Iceland was by all appearances unaffected directly by the rash illness.
Which makes sense. Because the disease is, as we've noted, basically a total wipeout. We would know very easily if Iceland had suffered a substantial infection -- because the Y90 population would be in the 100s, top.
So that means that Europe got hit, pretty much indiscriminately, while Iceland did not get hit basically at all. What was the difference?
If you go back and read the prelude, the difference was two days. Two days, at most, between when the government in Reykjavík made the decision to impose absolute quarantine on the island (combined, presumably, with quarantine on all visitors who had arrived recently), and when the other governments of the world began following suit.
What difference did those 2 days make? It was the difference between being more or less unaffected and total carnage. That is, 2 days later was too late. It didn't help, at all. No, nobody was dying yet on day 3 -- but they were all carriers by then. They were all walking around doomed, and didn't know it.
For instance, when Gøran is reading the newspaper and saying Scandinavia is likely to be infected in the next few weeks? The paper is wrong. Scandinavia is already infected. Except Dalsnes because it's totally cut off. By day 3 Denmark closes its borders, but it doesn't help. Denmark is already dead. The people on the ferry boat were by some freak coincidence a whisker ahead of the rash.
This is a lethal freaking disease. Virulent, incredibly infectious, and incredibly lethal.
And it's the problem with any of the scenarios we're talking about here -- if you weren't already shut down on day 0, you're all dead. All of you. Everyone.
If you're an aircraft carrier and you had personnel transfer in the 48 hours between day 0 and day 3? All dead. Floating tomb.
Remote, sparsely inhabited island with a bed and breakfast for tourists? All dead. Or trolls.
100 miles of fence that you couldn't patrol constantly? Sorry, someone got over. You and everyone on your side are dead now, too.
So from the point of view of imagining intact societies, we're faced with an interesting situation. In some ways it simplifies things. It doesn't really matter so much who has what equipment or training, or which kinds of geographies help or hinder human survival. It all really boils down to one thing: were you cut off from the rest of the world by day 0 or weren't you?
That's not to say there won't be pockets of survivors in odd nooks and crannies -- statistically we'd expect there to be a few hundred thousand people alive spread across the USA by Y90 for example, mostly on islands in the Great Lakes or off the Atlantic coast, or in remote areas of the Rockies. Maybe some of the smaller Hawaiian islands. And the pre-rash American military would leave a legacy -- but it's hard to see how there could be any kind of intact society. Look at Europe. The nations of the mainland are all gone. Completely wiped out.
In the end, nothing they had helped. Nothing at all.