RV,
Its is awesome that there are boots where you are that work to -50, and that you have back country to ride your snow mobile in, but such advice and statements are of limited benefit for folk living elsewhere. They do not help anyone in the UK, dealing with soggy, damp-cold UK conditions. Wellies or similar are very good here. Try to put yourself in Willcurrie's shoes...or shoes that would work for him
When I have been in Canada the army mukluks with mesh+felt insoles and felt liners were great, but not suitable for most conditions in the UK, even in Scotland. They could be made to cope with slightly damp snow, but not wet and mud. They would be impossible to maintain here.
I think that the greatest benefit of the idea of those far north boots is that they insulate foot and lower leg very well, and are pretty loose, not restricting circulation. Sitting in one position is going to be a challenge even in warmer conditions if your circulation slows and your position tends to cut off the blood supply to your legs. I can get cold feet sitting in my room at my computer for too long, and that is at a balmy 18C. How well insulated are your legs? Your feet are at the end of the circuit, you want to keep warm blood getting to them, so keeping the "pipes" warm will help.
All the advice on boots could see you sorted. If that still doesn't work, the next option would ditching the boots and sticking the lower half of you in a cut down second hand sleeping bag, like a body mitten. Have your feet resting on a chunk of foam to keep them off the ground. You can see guys doing similar things (albeit still in boots) for glassing for big game in the US and Norway, using things like the Jerven Bag, Kifaru Woobie / Slick bag and Hill People Gear Mountain Serape. No need to go that expensive though.
Good luck
