Hi Betsy, my house is built with a rubble trench foundation with layer of insulation on the outside of the foundation to prevent heave. I believe in MN you would need the apron to extend out from the foundation 2 feet, but I would double check. With a shallow trench foundation you are relying, I believe, and someone please correct me if I am wrong, relies on trapping the heat from the stable earth temp and the heat produced from the house. Even with some insulation you are going to have heat loss to the ground which is actually positive for those cold winter days when you are preventing frost from ruining your foundation. The rubble trench is also superior in that it wicks moisture away from under the house. No water, no frost, no heave.
No concrete slab is necessary for your construction of an earthen floor. Just level, put down 4-6" of gravel, use a moisture barrier, put down rigid foam insulation, do a base earthen layer (quarries have road base that works well), then do a finish layer of earthen floor that gets oiled once dry.