but
I thnk the coffin corner business is part of the airspeed problems. it would have been easy enough to stumble a bit closer to the regime called coffin corner if you are way off your airspeed.
stall speed is changed a bit under heavy G loading...and we don't yet know what the turbulence did to G loading on a plane that might have been flying too slow due to poor airspeed information.
if the airspeed was off 35 knots (unknown) and the flight envelope was 25 knots on either side...one could easily stumble.
I want to make it clear that in normal operation with normal instruments etc. FL350 was probably a normal choice.