Your other points are great, but I still take issue with adding dirt or something to the footprints. My backyard had over three feet deep snow in it up to the last week.

...This is a very good point with regard to 3 foot deep snow. The snow depicted in KBM's dio is barely above the sole of the soldiers' boots. That's maybe 2" of snow? This interpretation is corraborated by the depth of the snow at the edges of the tread marks. With that shallow snow, the snow would definitely be muddied by the compression of the men walking through it...