Bis47,
I do not believe anybody is calling into question the engineers troubleshooting skills, because in all probability no troubleshooting was carried out. You are trained to dispatch an aircraft in a safe and timely manner. If it is on the deck for a number of hours, you would start troubleshooting, If you are called to an aircraft that has returned to stand, you go to the MEL. If you cannot find a relevant section in the MEL for the defective item that you have, it means that it is either an non airworthiness item or it is a no dispatch.
It appears that the fault was diagnosed as a heater problem and action was taken just for that and the RAT heater CB was tripped. As I have said before, if any changes are made to any procedure after the investigation has finished, then the problem does not lie with the engineers. If all the procedures remain the same, then the judge will be stating that the level of knowledge and training of the engineers involved should have been enough for them to understand that the problem was that a relay was in the wrong sense, so causing the RAT heater to be on when it should be off and that the use and understanding of the MEL was incorrect.