I have a Wiggy's synthetic -60F bag and I put my Summer 0F bag (comfort rating 30F I believe) in it. I don't know that I've ever camped out at -40F. Most thermometers I've found stop at -20F (annoying as heck.) I haven't had any issues for two nights in a row with moisture.

Mickey mouse boots are a vapor barrier - which means wet socks (in other words bring lots of extra socks.) One of the biggest issues is drying stuff out.

Regarding the face issue - petroleum jelly applied to the skin generously helps. Actually keeping skin hydrated helps as well (i.e. lotion and keeping hydrated.)

Outer clothing does get frost - one of the ways to dry things is to put it closer to the body so the moisture is pushed out (driven out.) I'm not sure what happens if you don't get your outer clothing to dry at some point, as I'm usually only out for a night or two.