Is it possible that such a fire on MH370 could explain the incapacitation of the crew and PAX, and also the deactivation of the transponders?
Doubtful. SATCOM transciever was on, indicating AC power was available for the many hours after the aircraft went silent. The course changes made post-silence also suggest the aircraft was under the control of a thinking, conscious human. Finally, the transponder wiring, antennae, LRUs, and control head are literally on the other end of the aircraft from the tail-mounted ELT.