Thank you for posting this list. Extremely helpful even with typical caveats (everyone is different, hydration level, etc)
My winter set-up is similar and I have always wondered about the lower limits.
I couldn't have gone a third night with my gear regardless of temp unless I could have thoroughly dried out my quilts. The frost accumulation really starts to degrade the effectiveness of the down. When it stays cold the next day the frost/icing doesn't dry out on it's own like it does for three season camping. I was cold at 5am on the last day and had no choice but to get up early and start packing up right away to get myself warmed up. The big key when you get cold is to get active whether it be trudging through the snow or just going for a walk. My toes and fingers were numb and I grabbed a pair of glove warmers (felt like ice cubes for about 15 minutes until they started to work) and then I started packing up my gear, digging out snow stakes etc. and within 20 to 30 minutes I felt warmed up again. Amazing what body heat will do for you. Just getting the circulation going and general body heat will heat up the susceptible areas like hands and feet and you start to feel much better.