Hi! An update.
We opened up the floor and ceiling. The insulation had been done real shoddily so that was worth doing. There were huge GAPS in insulation. Like an insulation panel having been cut 4 inches too short! It was an enormous undertaking (even in a tiny house like this), but I am happy with the decision to tear all shitty work down and do it anew. Patching a bad job leads to chaos.
I lived several months in a shed, as the house had to be emptied. But it was summer so no worries there. As I had a great carpenter, we also finished the sauna and built a harvest kitchen (it's being finalised now)
The house immediately felt different. No draughts. No moist chilly air on cold autumn mornings. Gee, when I boil a cauldron of water, the moisture fogs the windows! And it stays warm.
Of course, this is no zero energy house, but it stays nice and cosy even now when the temps are around 10°F.
I am grateful for the hardship last year though. It was no joke living in a house with inner temperatures around ten degrees, but I needed it. Now nothing scares me. I feel enormous gratitude for destiny throwing this challenge at me and proud of myself that I stayed put and managed.