Yes the m90 is great. Slept in it some nights in the cold and it's amazing how well it performs. Although I have not slept in it below -10. The -50 you mention is amazing
Neither have I, but wearing everything else underneath a night in a snow cave should be surviviable. I have sat for 4 hours in an OP wearing the old M59 parka at -45 C; cold but survivable, and enclosed shelter should be better.
Drying wet cotton next to the fire in freezing temperatures is a trick I'm not very good at. Surely not if it's snowing. What is your preferred way of drying clothes next to an open fire?
Either in a heated shelter, or hung up "next" to the fire, not directly above it.
I would like to mention polycotton pants, they dry faster, are thinner and useable in a bigger range of temperatures (in combination with longjohns). They pack small and are easy to have as an extra pair. I'm a big fan of wool pants, but for a lot of activity the polycotton is better if temperatures are above something like -8. Ofcourse everyone knows them, but they were not yet mentioned in this thread as far as I remember.
For the warmer temps something like the G1000 over merino wool is ok. In really nasty thaw conditions I've wore just wool long johns and the outher whites. Not the best of wear, but far nicer than getting the wool pants wet either from the outside from melting snow or from the inside from sweat (the temp was +2 C, we were skiing only in the sense of "moving forward with skis on, on top of snow", I think I had a 10 cm thick cake of snow on the bottom of my skis).