the 'decel' light tells the crew that 80% of the commanded deceleration rate is being achieved.
your point remains very valid as there is no way of differentiating between the deceleration provided by the reversers and that which is being provided by the brakes.
OK, but there is no way that the reversers alone could achieve the 2.4 m/s/s required for the MED setting, and I say again, the deceleration achieved -from the moment the reversers have been announced "active" by the F/O and the 70 kt call, that is to say
before auto-brake application- is in fact superior to the requirement.
On the same subject, the influence of the reversers is limited, certainly not as important as the brakes'.
On the subject of that value of deceleration, had it continued, the airplane would have come to a stop some 12 seconds after the 70 kt call ( 42m/s diveided by 3.4 m/s/s equals 12 + ). that means some 252 m later than the 70 kt point.
Going back to my figures, the distance from 40 ft (rad-alt call to a complete stop would have been 1580 m + 252 m = 1802 m.
Though still a ball-park figure, it tells me two things :
1/- Their landing technique, all things going normal would have used most of the runway...
2/- As they were so close to achieving a stop on the runway, the excess speed they had was very low, which explains the survivability of the crash, even after the drop over the cliff.