"Can it's stall really SOUND like an overspeed? ... If it did, that could certainly explain the confusion on the part of both the PNF and the Captain, but the PF?.... I have difficulty understanding how he could have interpreted motions and responses to his inputs as an overspeed vs a stall. ...even if it did sound that way.
In an aircraft with unpowered controls, the pilot gets important tactile feedback re airspeed from how much force is required to move the controls. I doubt that an experienced Cessna pilot, for example, would mistake a stall for exceeding Vne in a light aircraft, even in the absence of external references or air data.
Obviously that's missing from this aircraft, although that begs the question of how control "feel" could be correctly added in the absence of reliable airspeed. Presumably Boeing has such an algorithm; whether or not it would have worked in this situation is beyond my ken.
But it does go to the problem of how far the pilot is separated from flying the airplane (as opposed to flying the computers) in 4th generation air transport aircraft.