Thanks a lot for your reactions. I think I will go for the no helmet option in Rovaniemi only. The rest of my cycling includes my helmet already and won't change :-).

Time to introduce myself as well then. I am Harold, based in the Netherlands, 46, married, 4 children. Started cycling on age 2 I think (as all Dutch do), but only took up mountain biking about 10 years ago. My world has never been the same since then. I found out soon enough short distance/short duration is not my strength, but marathons were. Up until now I have participated in a lot of marathons, but the most memorable are my 7 participations in Duchenne Heroes, a charity mountainbike event from France to the Netherlands. In total 700 km in 7 consecutive days, through wonderful terrain!
I have taken up fatbiking  about 4 years ago with the build of the first Surly Pugsly imported in the Netherlands (I bought it from a friend) and since then I have neglected all my other bikes I'm afraid. Next week I will get my hands on my new bike, a Surly Ice Cream Truck, with which I will participate in Rovaniemi 150 next year. Ultimate goal is to participate in the ITI somewhere soon, both distances.

I am grateful to have found such a rich environment of knowledge on winter bikepacking! Helps a lot with my preparation for my first winter race ever, the Rovaniemi 150.
Even though I have found a lot of answers yet, there still is one uncertainty on what to wear and that is the helmet. Usually I never leave home without my helmet when riding my bikes. But for winter biking and especially during long hours, I expect to need a combination of both a warm head as well as some form of headlight. Even though I can think of ways to fit a warm hat/balaclava etc underneath a helmet and fit some form of light on top of it, I am seriously considering the possibility of riding without a helmet. Basically because of the low speeds in combination with more possibilities for changing my head gear based on changing circumstances. What do you people think? Do I make a thinking error here? Is there definitely a need for helmets even on low speeds? Looking forward to your valued insights!
One final remark: my apologies for any grammatical errors, I am Dutch