One thing the video clip impressed upon me was the foam was falling in one direction, but the shuttle was moving upward in the opposite direction, so the force of impact would have been far greater than had the foam and shuttle been moving in the same direction.

Still the same problem: the foam probably wasn't falling yet, just no longer rising as fast as the shuttle was. It looks like falling because the camera is following the shuttle up, at say 2,000mph, while the debris had slowed because it was no longer attached to the pushing-up rockets and because of air resistance.

How much did it slow in that less-that-one-second? to -- what? 1999 mph? 1950 mph? 1900 mph? 1500 mph? That's what determines the severity of the impact.