@RTM Boy (on the subject of mid-air fire)
I understand that it's very difficult to ignite or sustain fuel combustion outboard the aircraft at FL 300+
But of course, the aircraft took some time to reach the ground, and soon passed into much denser air.
For example, TWA 800 fell with a "fire trail", the appearance of which was interpreted by some witnesses as an ascending missile. That flight's break-up initiated around 14,000 ft. I don't know whether the streaming fuel fire ignition source was determined -- it might have been by the explosion, but conceivably the heat of the explosion passed in a moment and the streaming fuel was subsequently ignited by another source.
If we imagine the Metrojet A321 breaking apart at altitude, and fuel streaming from a ruptured center tank, static electricity is a possible source of ignition (as the fuselage falls into sufficiently dense air).
In addition, with extreme damage to the aft of the plane, and the tail perhaps already separated, severed electrical cables might well have been persistently arcing in open air, where the arcing could come into contact with a fuel/air aerosol streaming from the center tank.
Probably, at least one engine was still turning, with its generator powering the plane's electrical systems -- arcing might have continued all the way to ground impact.