I kind of have(had) the same problem. The low temps are a nuisance, but not THAT much. I dry my flowers at around 11-15cº (13º average). I think the cold temps make them taste a little bit more "green" or "fresh"; I don't know how to describe it, but I really think it tastes different (maybe the Chlorophyll "dissipates" slower or something, IDK).
Humidity is a much bigger problem ofc. If the cabin is insulated enough, then go for a dehumidifier, don't even think about it. The place where I dry my buds is not thermal insulated either, but it is kind of a small place (~9m3) and the dehumidifier Works great there. The difference between dehumi and no dehumi is gigantic (really, at least where I live).
What I do now is: The first week I try to keep humidity at around 62%, and then I drop it gradually till around 50% by the third week. I let the flowers sit there as much as possible, usually around 2 months or so (13ºC and 50% RH).
Doing what I just described I got the best results so far. It's not optimal, but the smoke is not that harsh at all, in fact is quite pleasant. It can sometimes make me cough a little, and when this happens I just add a pinch of tobacco to my joints (70-80% MJ, 30-20% tobacco), which makes the smoking experience so much more enjoyable and smooth (tbh I always add a Little bit of tobacco, even if the MJ is great and smooth. I just like it). If you don't have 2 months to dry and kind-of-cure your buds, I guess you can go for a more agressive approach, starting with 55%RH and dropping it to 50% by the second week.
On a side note, I'm not sure how beneficial the fire is. It generates smoke and ashes, which I think have the potential to mess your buds a little by contaminating them (the smell and flavour of the smoke and the floating particles of ash).