What is the enforcement difference between a PSK foul and a foul during the return?
PSK is an exception to the basic spot for a loose ball play (before the kick ends) which would otherwise have previous spot enforcement.
The basic spot for a PSK foul is the end of the kick, unless the foul is behind the PSK spot. Then it's the spot of the foul.
A foul "during the return" (after the kick ends and R has possession) is handled like any other COP play.