Hi Lonewolf_50;
There are some (who know their aerodynamics) who have done some work on this and who believe that the airplane could be recovered, even from quite low altitudes, (thicker air). The horizontal stabilizer was never fully stalled and the airplane demonstrated the ability to pitch down when ND stick was held those rare times in the ND position. As you know I've done this scenario in the simulator a number of times and the result is always the same; the elevator is effective, the stab returns from +13deg to a more normal -2deg or so which all helps to unload the wing, reduces the AoA and unstalls the wing. I wouldn't say they were done by 15000ft but perhaps by 8000ft or so, (which I have not tried; high up, it takes about 15000ft on avg to unstall the airplane - lower down, less).
I think they could have made it with room to spare. Also, I disagree with the use of the term "deep stall", which only applies to the BAC-111 and DC9 configurations. Discussed at length and not about to be re-argued here.