" ...when I got home, I noticed my mid-section was very cold/numb. Scary."
I would not worry too much about it, I've had that for many years now. As I've mentioned above, pure wool is always the best inner layers for winter cycling, as it keeps you warm when wet. Some time ago I bought myself a pure wool boxer short, that helped. I haven't looked further into the matter, but I believe that you could find possible solutions in those shops that specializes in clothes for people with rheumatic problems - I've seen battery-heated jackets and trousers, as well as soles for your shoes. Well assorted bike/sport shops should also be aware of the problem.
Another way of coping with this is actually to gain weight during the winter. Fat acts as a protective layer against cold. All those munchies during dark winter months is actually your body attempting to adapt to the climate. Just look at the chunky shape of the Inuits, Mongolians or Siberians.
"Are there any opinions on riding a fixed gear in cold conditions (-30 C and lower)? On the one hand, the continuously spinning pedals wouldn't allow the rider to put their feet down to make a turn, but on the other hand, I've heard from local fixed gear riders that that type of bike is preferable in very cold conditions, as there are fewer parts to fail on you. For example, a friend with a newish cross bike was unable to shift properly (brake shifter combo style) in only -10 C."Even though I have never used a singlespeed during the wintertime, it sounds very right to me. With that cold, it does not take more than a few drops of water at the right place on my derailleur before it becomes difficult to shift properly. For instance, leaving the bike indoors so that the snow melts but not long enough to dry up again before taking it out, will pose a problem.
A singlespeed will also save you the trouble that you face when commuting long distances in a lot of snow - the snow has a tendency to pack itself around the derailleur and sometimes make the movement of parts difficult. Just keep in mind that the salty sludge on the street also eats up expensive fixed hubs ...