Before they replaced the thermostat, they should have ohmed the coils. if that was the problem, they should not have replaced the thermostat. Do you have pics of the coils on the floor, before the drybed was put over it? by scrutinizing those you may be able to find location of existing problem, possibly fix it there. to be honest it is usually, at the wall/floor joint, where the wires go from the floor up to the thermostat. If the wire was left too much out it will overheat in this area, from no substrate to absorb the heat, wire just overheats, and burn itself in 1/2, or it could have been pulled up too tight and crimped in the wall, making a minicoil that overheated, or possibly a steeple was put in too tight, I would take my chances with drywall demo between floor and thermostat before i started on floor. May just be repairable, if its the main wire, before the resistance wire starts.