If the polar bears and the whales take care of each other Inuit could survive on fish pretty well, right?. For a while at least.
I actually doubt that marine mammals could become infected, maybe polar bears would be immune as well.. trolls and water simply don't go very well together, in mythology they're almost as vulnerable to water as they are to steel and iron, hence why waterways are considered 'blessed' in the comic. And if whales were indeed able to be troll-ified, I doubt there would be much in the way of sea travel anymore. Iceland would not be safe, the whales would infect the seals and the safe zone would be in constant danger of being contaminated.
Anyway, my own two cents... almost all of the mainland is dead. Yep. Sorry. It would need some sort of natural protection to keep the trolls out, like Scandinavia and its cold climate. I bet there are survivors in Russia and Canada, maybe volcanic islands or other isolated islands.. but very little else. The only reason why people were able to reclaim Mora, destroy old cities, try to reclaim Copenhagen, and make Finland a bit safer, were because of the trolls hibernating, being sleepy, vulnerable sitting ducks that could be struck first. That sort of luxury is not enjoyed by most of the mainland.
I suspect that parts of the UK might be populated. They have plenty of islands, harsh and cold winters, and a lot of livestock and farmable land. On the other hand, all that overcast weather... hmm....