I am building a workshop in the Catskill region of NY state.  The design will incorporate a 4" radiant hydronic slab and have frost-protected shallow foundations.  The building is 24x36 feet.  The 8"  deep x 20" wide footings will be 8-12" below the slab level and there will be min. 18"  of backfill (up to 4 feet in back)  There will be lots of XPS on the outside vertical surfaces down to the footings and some horizontal skirt insulation too.

Thinking it would be easier and less expensive I kept the footings all the same level.  However, we hit ledge at the back wall and so kept the top of the footings at that level.  Also, I wanted to keep the concrete walls at a reasonable height and not have to form higher than needed (I want 10 ft. ceilings)  Thus the footings are interfering with my slab placement, which I want to be as low as possible.

I'd like some help designing the junction of the slab with the inside wall. 

One option would be to just place the 4" slab + 2" of xps right on top of the footing.  I'd lose 6" of ceiling height and need to bring a lot more stone to bring the slab and driveway up that extra 6 inches. 

To go lower, the slab needs to either key over the footing, or sit flush with the top of the footing.  Both options will leave me with the 6" wide inside perimeter of the footing either exposed to the interior or bridged to the of slab.  Remembering that it's only 8" down through the footing to the earth (actually several inches of crushed stone over clay) what's the best way to insulate the slab?

Why not set the wall height from the top of the slab rather than the top of the footing? Then you can plan for as much insulation as needed.