I can't comment on teh heatsheet, but mylar (aka "cheepie space blanket") does not make a good ground sheet, particularily if it's cold - it doesn't block conductive heat transfer.

I took a youth group camping and wound leaving my tent behind (smooth, I know). So, I started with a small tarp for moisture control, then unrolled my GI sleeping pad, then a "cheepie space blanket", then my sleeping bag then my poncho to keep the dew off. That was the most comfortable I had ever been on a cold weather camping trip. How cold did it get? Cold enough to freeze most of the water in a 5 gal jug.

MrJim