Forgive me Ralph, I must have a splinter in my eye preventing me from seeing this situation correctly.
As I understand the situation, K attempts a FG on 4th down, but commits a (live) loose ball foul (at the snap). FG misses, so R chooses to decline the penalty rather than accept it. If the penalty wre accepted, the 10-4-2 exception provides an option to enforce the penalty from the succeeding spot (R's 20 YL (touchback) or 5 yards further back.
By choosing to accept the penalty, doesn't R, in selecting the penalty yardage, give K the advantage of "replaying the down", which would maintain K's possession and the opportunity to repeat 4th down, as they see fit, giving K another opportunity to repeat their FG attempt, rather than ending K's series (on downs) and taking possession of the ball to start a new series of their own ?
What am I missing?