In terms of what is missing from SSSS, the biggest question in my mind is, what happened to the books? It seems from some of the dialogue that the books in Denmark are valuable for the information in them, not just as rare souvenirs. Yet Iceland was certainly never overrun, and Bornholm probably wasn't, or wasn't completely. Even Dalsnes and Sollerön seem to have survived intact. All of those places have plenty of dead-tree books (well, maybe not Dalsnes). According to Google, Iceland has seven libraries just in Reykjavik, not to mention book stores. The knowledge from all those books should be readily available, but apparently isn't.
A second question is, how could Christianity have been so completely forgotten that Reynir doesn't even know about it? I brought this up in the comments once upon a time, and was accused of being unwilling to accept that my favorite superstition had been forgotten. No, I'm not a believer, but I'm certainly aware of the existence of Christianity, its beliefs in a general sense, and the appearance of its churches. It's been a huge influence on Western culture, including in Iceland for a thousand years according to Onni. Reynir's great-grandparents would have known all about it. Reynir should know something about it, but he doesn't.
My personal answer for both of these questions is that a deliberate effort was made to wipe out the old, technological civilization. Maybe there was a belief that the Rash was intentionally created and spread, by technology or by Christians or both, or maybe there was just a sense that society had utterly failed and people struck out in desperation and anger against the things they could reach. But those questions aren't addressed in the comic, and I suppose never will be now.