Normally you and especial the two experienced pilots would have recovered the bird within 500 to 1000 feet even from a power-on or power-off stall. And it did not crash in a flat spin. There is reason to believe that they had those altitude margin.
Spinning is a quite complicated "maneurvre". It takes an initiation phase before you are in a stable spin and terms like "flat spin" make sense. This phase may already use up 1000 feet in a large aeroplane. During the first phase of a spin it is not unusual to fly inverted or in a steep dive before a stable equilibrum between all erodynamic and dynamic forces is established. Check
this picture of the early entry phase into a (fatal) spin for example. In the following second the aircraft will be in an almost vertical dive still acellerating the rotational speed before entering a stable spin after somr 5-10 seconds. If you run out of altitude at that moment, the result looks like in
this picture. Which matches the Junkers wreakage quite well.
The recovery altitude given in the handbooks is between starting countermeasures in a stable spin and the full recovery. The altitude between stall and stable spin can not be given, as there are too many different scenarios how to enter it, for example the actual bank angle at the time of the stall.
anyone with even a rudimentary knowledge of aviation could easily come up with several obvious scenarios as to how the aircraft came to the fate it did
For me there are basically two realistic scenarios, stall and spin or elevator control (elevator trim) failure. Although people talk about "high speed" I think the low fragmentation of the wreakage does not confirm that, Impact speed must have been well below 200 km/h (compare it with the oskosh mustang, where only small fragments were left and the aircraft lierally "exploded" at impact purely due to kinetic energy). So for me the stall and spin theory, linked to relatively slow inpact speed, makes more sense. Although a stable spin has never been established due to low altitude, so it is stall and spin entry... A former BAZL expert thinks the same.
The sad aspect is, in this case the investigation has nothing to find. The wreakage will tell no story, there will be no "smoking gun". There will be no corrective action. It happened a thousand times before, it will always happen.