Had I packed for 0 instead of 20, I would have had gear good to -10 instead of 10. The catch is that you have to carry that gear.
Perhaps the underlying moral to the story is that elevation forecasts are not very accurate. When making a three hour drive to get a to a winter backpacking trip, bring alternate layers and insulation to repack before starting. Another extra level of safety would be assuming that the shelter would be all but inaccessible. The problem with all of those levels of safety is that they add weight and take up space. Eventually you will be so safe that you cannot carry all of your safety in one trip
My guess is that the best compromise would have been to take an extra top and bottom layer than what I did haul in. That probably would have been enough to have avoided most of the issues.
As far as footwear goes, the Neos overboots that I had were more than enough to make the trail runners I wore under them warm. They provided insulation, waterproof, traction and a larger footprint that provided some flotation on deeper snow.