I agree with Warrenkicker. The momentum exception does not apply to the OP because R1 fumbled the ball before it became dead in the endzone. The post-scrimmage spot for PSK fouls is the end of the kick, in this case R's 4. R's ball, 1/10 @ R's 2.
Had R1 NOT fumbled the ball, the momentum exception would apply. But since there was a foul, the basic spot would still be the end of the kick (not the 20 due to the touchback), and enforcement would be exactly the same.
So the fumble in the endzone is a red herring.
