In extreme cold I choose the waist belt pants with a good long close fiting parka. It is to big a chill to take off the parka to get the bib straps undone and sorted out when nature calls for #2. It is also nice to have two layers on my lower back for sitting by the stove.
I travel with a light woodburning cookstove/chimney (2 lbs). I do not try to heat up the uninsulated tarp shelter to room temperature. I therefore do not like taking all my clothes off to get in and out of the sleeping bag. So I use all the clothes I carry to stay warm when I sleep. No one has posted any dire consequences from sleeping in ones day clothes.
I am not really sold on the layering thing, it is easy to unzip the parka or flop the hood back. A hat under a hood bothers me and when I put the hood back I may get some free vitamin D from the sun on my bald head.