It was/ is impossible for a 757 or similar type jetliner to travel at anything like 520 mph at 1000 ft. and lower.
500 mph [or thereabouts] is in fact the speed routinely used [for fuel economy purposes] by airliners at their cruising altitude of around 35,000 ft.
Furthermore, the planes engine air intakes are physically unable to handle the increased amount of [denser] air they would encounter at 1000 ft. and below at the alleged speed of 500+ mph.
Also, the plane , regardless of whether it was piloted via remote control or by a real life pilot, would never be able to perform the sharp diving descent and swerve first right, and then left [from the "pilot perspective"] that is clearly seen in the CBS "live" clip of Fl.175's supposed last 16 sec.s . :