While my sub-freezing camping experiences have been limited (note the screen name...
...my coldest night was 21*), I've read of several folks having trouble with the foot box of their down quilts/sleeping bags collapsing due to moisture accumulation. So...the frozen foot thing? It's not just you. The thing that's oft-recommended by folks is to wear bread bags (hear me out here...I know it sounds crazy) over your feet, on top of a pair of very light, thin socks (so to not have the "plastic" feeling against your skin; think dresss socks here), in order to prevent the moisture from your feet collapsing--or freezing--your foot box. Then wear your normal wool sleep socks over that.
Another thing that's recommended to avoid condensation on top of your pad (due to it being a vapor barrier) is to have it inside
the foot box of your bag/quilt. This prevents it from getting cold enough for water vapor to condense on it, thus causing that vapor to pass through your breathable insulation and out into the surrounding atmosphere (in theory; often, it will wind up freezing in your insulation if the surrounding area is too cold).
You may or may not be able to get down to 0* with 20* rated stuff and extra clothing; I know that I can stretch a 40* system at least to freezing with appropriate clothing...but it's heavier and bulkier than simply buying another set, and I wouldn't recommend it for true comfort. Simply warm enough
for me to get some sleep; not warm as in, "I was toasty last night," warm. YMMV, though.
Hopefully, some of the folks who do real deep cold camping will chime in here a bit, but that's what I've read and experienced over the last year or two...