Ed,
The issue here may well have had no different result if the wrong speeds were used. ie the acceleration to (your speed) of 100 KIAS would probably have been within a few seconds of normal.
The problem could have been trying to rotate the aircraft at much too slow a Vr. The aircraft just wasn't ready to fly. They would also have been above the "wrong" V1, and we are all trained to go after that speed.
As has been said before, it is an extremely dynamic situation and valuable seconds would have been taken in wondering why the aircraft wasn't responding. The rotation process over say 5 seconds would have consumed well over 1000 ft in itself, and probably much more with the drag of a continuous tail strike.
The hindsight committee will no doubt say they should have lowered the nose to the correct pitch attitude and immediately apply TOGA thrust. The crew on the night was faced with a situation that they had never ever seen before - so comment after the fact is of little consolation to them.
Time assessment on very heavy take offs, once again, would be a subjective process, even at 100 KIAS. I can't see authorities approving such a system unless it was able to be calulated for every take off. That would also require inputs for possible dragging brakes, airframe deterioration etc etc etc.
It's just not possible to do it accurately.