How deep were the 6" holes that the contractor drilled and filled with concrete? I'm thinking to drill much smaller holes, maybe 1 1/4" diameter, through the concrete at strategic places and pound in 3 or 4 foot long pieces of 1" diameter rebar down below the top surface of the concrete and then fill in the holes with concrete level with the top surface so the repair isn't visible. Hard to believe that wouldn't stabilize it. It would be relatively inexpensive and something you easily could do yourself.