autothrottle issue?
the issue with the autothrottle seems plausible -
fully configured at 3nm in approx. 1000ft with F30 and being distracted by heavy training on the flight deck on final approach, speed starts dropping and nobody realises that the A/T is off. aircraft gets upset and descends below glide directly into the wake of the preceding 757 (observed by the pax as turbulence), which could make a recovery well below 500ft impossible...