You had this problem last year, if I recall correctly. Being the owner of guitars that all have unfinished maple necks, I say again: you have to get the humidity in that room between 35-40% and keep it in that range. I don’t care how drafty the room is, you can do it. And it’s not the insulation or leaky seals (those are probably helping a little bit), it’s the dry heat being pushed into the room that is driving the humidity down. Buy a humidistat for $10 to $20 and minimize fluctuations as much as possible. Finished necks will react more slowly, but as you can attest will still dry out and start to fight against the non-organic material in the neck—truss rod, neck, nut, etc.
You are correct about the humidifier too. Buy one that is evaporative. Depending on the design and the content of your water, you might develop some scaling issues from the stuff left behind. There are some quick solutions, if you google. I personally use good ole vitamin c that you can buy in bulk on amazon. But I believe some people also use vinegar and other natural solutions.
Put open containers of water on your heat source as well—this is a free solution that can help a little bit but will not be a stable solution. An old pot on the radiator is that’s what you have; a glass on the vent if you used forces air. Whatever it is, the water will evaporate into the room slowly and naturally.
Also, the same cuts the other way. If you get a ton of moisture in the summer because if AC, the you need a dehumidifier. Our house for example can have humidity over 60% when temperatures rise. This is actually harder to control, in my opinion, because even good units have a tough time removing humidity from cold air and keeping things under 55-60%, but a good dehumidifier can keep things right where they need to be.
Hope this helps.