No advice sag, I really feel for you! We have the same floating slab requirement.
The land where I built had been contoured/graded with earth-movers before sub dividing and building. Code requires foundation to go below the original frost line. My attached garage has 9' (foot) walls down to foundation. A big plus is the garage hasn't settled vs the house.
Years from now this garage episode will be one of the tales grandpa tells along walking 2 miles to school uphill through the snow. The grandkids will roll their eyes and say yah right! LOL!
My '64 1/2 vert. Ordered May '64. D code 4 speed, handling package, caspian blue, accent group, Ford blue manual top.
'68 vert. driver. Owned since Apr '78. C code AT, AC, PS, P disc B, PT lime gold, standard black interior and top. NOS RF fender and left quarter.New top and folding glass.