A fire in the hold would surely elicit response/record on cvr/fdr.
No.
I understand that it's very difficult to ignite or sustain fuel combustion outboard the aircraft at FL 300+
Irrelevant.
The fundamental assumption these posts make is that the instrumentation was working properly and being monitored properly. We don't know that. All we know is the that the flight parameters were normal.
It is possible that fire happened in the hold that went undetected or unnoted and by the time it right FL 300 damage was done.
I'm not saying that this is what happened; I still think a structural failure is more likely. But none of the evidence produced so far categorically eliminates the possibility of a fire being the proximate cause of the crash, the intransigence of the mods of this board notwithstanding.