You can't have PSKE unless R has possession of the ball at the end of the down.
So from your description, nothing to do but take it back and rekick.
It took me a while to get a mental image of someone on the ball but not in control.
Recheck the requirements for PSK Fouls/Enforcement ... it says nothing about R having possesion at the end of the down.
According to the OP, the kick was not possessed by R or K and the ball was loose when the IW "ended the down" (and kick). Under normal IW procedure during a kick, we would replay the down (4-2-3a). However, we now have a penalty to consider and if accepted, it will take precedence over inadvertent whistle administration (4-2-3d).
The foul by R happened beyond the expanded neutral zone, before the kick ended and K will not be next to put the ball in play. PSK requirements are satisfied. The end of the kick is the "IW spot" and the foul is enforced from the spot of the foul if behind the end of the kick. If the foul is beyond the the end of the kick, enforce from the end of the kick.
"You can't have PSKE unless R has possession of the ball at the end of the down." Something to think about ... Does R have possession of the ball at the end of the down if; the kick goes out of bounds, the kick is downed by K or the kick ends in a touchback?