1. IF it worked for the first year, then your Inverter powered the defrost cycle OK.
2. What do YOU think the AMP draw is when on Defrost cycle? Hint. It is not that much and if you have a 1500 watt(capable of 13 amps) Inverter or larger, the amp draw will not be able to cause the Inverter any problem. The average defrost heater is no more than 600 watts(5 amps), some are smaller 400 to 550 watts.
3. With Electronic controls, you cannot turn the Defrost cycle OFF. Even OLD style mechanical controls you could not turn the defrost cycle off but you could easily disconnect the old style defrost timer which would stop the defrost cycle.
4. What you can do is access the defrost heater element or the defrost heater connections on the control board and install a 2 wire toggle switch to stop power from getting to the defrost element when the electronic board says to defrost.
5. As to your complaint about Tech Support. THAT is for qualified Whirlpool Service Center technicians and besides, NOBODY, will instruct you on how to modify the controls on your appliance. LIABILITY issues. Doug