1. They might be very hard to find, but the sabot pieces would be left behind at the scenes of the shootings somewhere just ahead of the shooter's position. I think that the investigators that are getting cadets to police the area would have come up with a few sabots by now if your theory is true -- assuming they're smart enough to know to look for them.
2. Small rifle bullets don't survive bullet traps well, even the water tank kind. If they do survive, they might not be very reloadable. If they are in enough shape to reload, they wouldn't be very accurate. Not accurate enough to hit someone in the head at the ranges this sniper is probably firing from.
Clever of you, though.