Just FYI, for anyone that wants to do a floor register, they do make fire dampers for this application... 3 Hour Radiation Damper - 2701
I have a walk out basement which means 1 wall is exposed to the out side air. It is a newer house and insulated well. Out here in Wyoming one thing is for sure and that is the wind is going to blow in the winter and blow good. I do not heat my basement and have never seen it go below 48 degrees. I have a hot water heater down there and a full bathroom in the basement and in 7 years have never had a problem with freezing. Do like you said and watch the temperature down there and see what it does. I think you will be ok with all 4 walls under ground.
