Please bear with me, I do know anything about the aircraft and it's systems, what I do know has been gleaned from this thread. It is not clear how the engineers disabled the RAT heater, from the diagrams posted earlier if it was disabled from the ccb this should have have not affected the other services from the relay R2-5. Much has been made of the aircraft being "in flying mode" causing the heater to be on, surely if the sensors thought that the aircraft was flying the TOWS was redundant as it was already "in the air".
If the procedure to check the TOWS before each TO was considered to be essential this should have been in the Flight Manual, if not, it cannot be considered to be mandatory.
Having been involved in the compilation of MEL's, they are considered to be a guide to acceptable deferred defects (ADD's) where for instance if an engine oil pressure guage is u/s:- as long as there is an alternative indication of low oil pressure, such as a low pressure warning light, it is acceptable to fly until a replacement guage can be installed, the reasons for the fatal flight appeared to be acceptable as icing was not to be expected during the flight.