NCAA Free Kick Play: Ball comes to rest in end zone...
 

With the wrinkle of it being caught first, in the end zone, by R1. R1 catches, turns and tosses to referee, who lets it drop to the ground, inbounds, in the end zone, and it comes to rest there. Neither K nor R move to cover the ball, and run to their respective sidelines.

Ruling?

Guessing here...
If the ball has not left the end zone, then the ball becomes dead by rule when it is not moving, on the playing field (or is it just field-of-play?) and nobody is making an attempt to recover it. Since the ball became dead in the end zone by this method, and the force of the kick--not R's actions--was what put it in the end zone, then it's a touchback.
Any K player who was on the field of play at the RFP can make an attempt to recover it for a touchdown, since the ball was possessed by R and is now a fumble, instead of a muff.
If R had caught it between the 5-yard and goal lines, and momentum rule was in effect, it could be dead, but the ball would be placed at the spot of the catch.
Initially caught beyond 5-yard line and carried into the end zone where dropped--Safety.

If the "toss" was forward it would be an incomplete illegal forward pass. The penalty would result in a safety.

If backwards I agree that it would be a touchback.

...then didn't you hear an inadvertent whistle? I'm sure I heard one.

Yet another reason for the WH to make sure he's behind the receiver.