but we don't really know if the airplane was unable to recover from a stall.
Nor do we know that it could have been recovered from the stall induced by the PF. What we do know is that there was no procedure (certainly no tested procedure) known to the pilots for safely recovering from a fully developed stall. Obviously getting the nose down and unloading the wing - but for how long? And, if there had been a procedure, wouldn't it have used airspeed as a guide to when to start pulling up? I am skeptical that even a pilot more experienced in hand-flying large transport aircraft than were the PF and PNF (the captain of AF 447, for example) would have been able to fly AF 447 out the stall induced by the PF without breaking something, especially in the absence of reliable and trusted airspeed information.