I think to sum up the results of the last of an experiment.
Exothermic reaction was still, but very weak. After removing the burnt spiral with fuel cell was discovered discharge of thermocouples with a piece of cement by about 2 mm from the surface of the reactor. On the second empty reactor, coil and a thermocouple were whole, there were only small cracks on the surface of the cement, once again, the material, the length of the resistance spirals were completely identical.
Suppose that the difference in temperatures in the range above 1000 ° C reached ~30%, the reason for that was a defect of fastening a thermocouple on the reactor with fuel to cause rapid combustion and a spiral (1), while the other remained without evidence of deterioration (2).
But it's the thought, intuition.