OK since you are referring to the similar climate and basically same stuff as we use in Norway here is what I use here.
You cannot use one sleeping bag all year round, sorry you will sweat to death and overheat in a winter bag during the warmer months. You are stating bug in, and then the question would be back to you, will you be in a cabin with stoveheat or similar? In that case any 3 season bag will do. If you will stay in without the luxury of stove heat or anything you need a winter bag for the winter months. Since snow creates a lot of humidity I prefer a synthetic winter bag, you will have bulk, you will have weight but since that is not an issue you gain a bag which will be hot even if it gets damp or wet. A down bag looses the ability to hold warmth when wet, and are pretty reduced even damp. So a synthetic winter bag should be on the list.
My winter bag which I sleep out with in the mountains of Norway during winter months is an Ajungilak Tyin Winter, it has scored best in test of several mountaineering magazines in Norway as best winter bag, with best tolerance of humidity. I love it and can sleep well in it below 0 degrees Fahrenheit with a silk liner inside, if you add a fleece liner you can go way lower than that as well with it. Great winter bag BUT I could not use this in summer times it would kill me with heat.
Again for the warmer months I would go synthetic if space and weight was not an issue, same reason as above the resistance to humidity is far superior and well worth it. I would go for a bag which could handle heat, so a low scored temperature rating would be great as a supplement to your Tyin winter bag. I would even go as far to say a square one you can zip all the way open like the North Face Dolomite 2S sleeping bag would be great during summer time. Then you would have a great bag for winter and cold and an excellent hot weather bag as a supplement.
Personally I have 3 bags, one winter bag, one 3 season bag and 1 summer bag. My summer bag is a down one because I move a lot during summer months fishing and hunting so weight and size matters a lot. I use the winter bag solely for winter, my 3 season for early spring and late fall, and my summer bag for all else.
So
Winter Ajungilak Tyin Winter
Early Spring, late fall Mammut Kompakt 3 season
All else Sea to Summit Spark spl
And always remember to use a liner, I use silk liner always and sometimes in the winter when its freaking cold I add a fleece liner and sleep well all year round. The liner is easy to wash and keep clean and protects your bag from you.
That works great in same to even higher latitude in the world.