The venting issue seems to be a significant one to prevent fogging. I've got a pair of wraparound sunglasses and they fog up rapidly once my speed drops below 12km/h, I'm labouring uphill, and the ambient temps is 0 to 5 deg C.
On my Nullarbor ride some years ago, I stayed at a backpackers in Esperance in south-eastern Western Australia. Another cyclist, a guy from Sweden, was there. He had done an apprenticeship or somesuch with the Swedish airforce, and had purloined a visor from the helmets that the pilots wear, and adapted it to his bike helmet. Looked bluddy good, actually. Full face protection, in this case, from sun.
Just a suggestion to see if something can be picked up for surplus and adapted. Would seem to provide the ventilation to avoid a degree of fogging, but at the same time protect eyes, mouth and nose from cold air.
I've just had someone stay with me from Canada, and I was regaled with all sorts of stories about winter riding. I don't envy living and riding in those sorts of winter conditions.