Not to try to rock the boat, but I have some issues with the climate, sans magic.  The 6 month darkness on Earth is truly only at the north pole itself.  I lived 12 years in Alaska, and spent 10 weeks in Barrow, north of the Arctic Circle (summer, fall, and winter).  The winter is brutal, and almost all mammals that can migrate have - caribou south of the Brooks range, so not near the Arctic coast.  And they return annually to calve, since the annual summer provides tons of plants on the tundra.  
In Anchorage in December, Astronomical twilight starts and ends roughly 3 hours prior to dawn and after dusk.  So even with 5 hours of true daylight, you still have 7-8 hours of usable light.  In the summer of course, the sun goes down each day but is sitting just below the horizon so it doesn't get dark/dim for at least a month on either end of the solstice.
In Barrow, it's admittedly stranger.  In the winter you have a twilight that lasts roughly 10 hours a day in December.  True the sun never actually comes up, but is sitting just below the horizon, so you actually have some 'dusk' light to work with in the middle of the 'awake' period of 4 hours or so.  It's weird, but I've seen it - we would joke that the 'dim' had arrived.  Summer of course it is 24 hour sun for like 2.5 months and probably doesn't get 'dark' for at least two weeks on either end.
https://www.sunrisesunset.com

The tree line is another potential issue - In Barrow, no trees.  Trees are like 200+ miles to the south.  Comparing Anchorage itself to the 'Coastal Mainland' range, the winter temperatures averages aren't too out of line, but years 5-9 would be low if compared to months.  I like the concept, but the growing season requires certain daylight and heat for crops, otherwise, no barley for beer.  Interior mainland - maybe like going into Denali.  I'd think years 5-9 would be low if compared to months, but not like you're growing crops there. 

Just my 2 cents.

I don't think you can impose mechanisms into a fictional setting from a non-fictional one.

Hyperborea for the most part has an internally consistent environment. I don't think one should worry too much about life finding a way.

Having communities planning for Nightfall totally makes sense. I've been adding underground granaries to towns of any size. Those are easier to construct than any above ground structure in my opinion.

What happens in the uncivilized parts of Hyperborea? Who knows, that's the makings of a great adventure I bet.

You're not even talking about the presence of predator gigantism, which implies huge numbers of prey animals, all of whom gotta eat! 
I say there is a diffused luminous radiation (ala Lovecraft's "red-litten" N'kai or "blue-litten" Kn'yan) that seeps up from the vaults of Underborea, allowing plants to survive without sunlight.

Not trying to draw a line in the sand at all.  Again, suspending disbelief over the big items (everything Brock Savage noted) that don't impact the day to day life is easy.  I just can't suspend disbelief on a 13 year year with regards to animal/crop/plant growth and health. Again, not applying my standard to anyone else.

Kevmo wrote:

I just can't suspend disbelief on a 13 year year with regards to animal/crop/plant growth and health

Also you have to consider that Hyperborea also is rumored to have a considerable UnderBorea, so they live an function without light all the time, so life can exist without lite for extended periods of time, also think of the possibility that even though there is no visible light, there may still be Ultraviolet, Infrared or other spectrum energies in Hyperborea that are possibly always on. Though there are a lot of Native to Old earth like plants and animals, they may have adapted or blended with other Hyperborean native plants to adapt.  Or Hell, it's a kind of magic world...