One theory holds that in this very tall 50k' storm, there may have been a hail "nursery" that they flew into. In theory, all the pitot tubes from one side could have been damaged or blocked but not the other side.
Airspeed is critical for control at that altitude and on that Airbus, cannot exceed .8 mach. If auto pilot erroneously increased air speed, the plane would quicky stall and then spiral dive.
Late at night, one pilot asleep, computer dampening any needed correction, a layering of faults. That is the key in accident investigation, unforseen layering of problems.
Pilots are warned not to believe disagreeing speed indicators but switch over to their backup GPS speed indicators. If that wasn't done, and pilots increased speed manually by mistake, a nose up stall.
When to believe and not believe instruments is the critical judgement of pilots and the cause of most accidents. Flying at night, autopilot, no visual references, sleepy tired.....pilot error.
One issue I have with fly-by-wire is that there is no "push-back" from the throttle levers, no tactile sense of turbulence or engine strain.
You are correct about the inertial guidance and GPS back up systems, they are available....if the pilots use and believe them.
A stall occurs when the airspeed is too LOW, not when it is increased. If the airspeed exceeds Vne, the plane will start to break apart, due to the air-flow induced forces on tail and/or wings.
Stalls are caused by LOW airspeed or excessive wing-loading ("g-forces").