Yes, you are right about the wood gas; that is what I was talking about; further information here:
https://en.wikipedia.org/wiki/Gasifier. I don't know how your lawmakers would rate the efficiency of such a device since it generates heat for you while also creating fuel for you which you can then use to generate more heat or electricity.
I think it would be a mistake to think that without the sun, the thermal mass would have no benefit. If it is cold outside, you will need to heat your home. The insulation that you have in your home right now is intended to prevent (as best as possible) the heat from 'leaking out' of your home. Once the heater is turned off, the existing heat will slowly leak out of the home, which is a complete loss. The idea of thermal mass is to store heat. So if you are heating your home, you are also heating the thermal mass. Once the heater is turned off, the thermal mass releases the heat back into the home, so all that energy used to heat the home is not completely lost. In this example, no sun comes into the picture, but the function is the same.
With the Earthship, the design is intended to take any advantage of the sun that is possible. The thermal mass works no matter what, but if you can face all windows toward the sun so that you can take advantage of what little warmth it has to offer in the summer months, then you are ahead by that small fraction.
Of course, a thin earth wall will not help you as much as a thick earth wall... especially in a climate like yours where you have snow on the ground for so many months... but 2 or 3 meters of earth would keep you well insulated from that. With the Earthship design, the window side is multiple layers... outside windows and doors with multi pane glass, an air space, and then inside windows and doors with multi pane glass... so the insulating ability of the window side is significantly better than most standard houses which have only one multi-pane window between the outside temperature and the inside.
My point here is that I believe based on the Earthship design, it is a significant improvement in efficiency even in your climate, even without strong sun available nearly every day, compared to the standard construction approaches available in most areas.
I do understand what you mean about deciding to adjust the house for better efficiency or demolishing it and rebuilding. It can really be a difficult decision, especially if you also have to work to get income and if you have no other place to live. In my case, I think my house needs to be demolished because the amount of effort to improve the efficiency would be much greater than simply rebuilding (I would probably reuse many of the materials in the new home). But I would need to find a place to live during the build so that I can continue to work since the build will probably take ~2 years.