The 5 mtre (goal line issue) issue is common sense since taking one a footfrom the line would be almost a nailed on try. However that does not apply with a Penalty near the touch line. The basic principle is the scrum or the kick is where the offence occured (unless there is a good reason to change or, of course, "options").
I can see some merit in coming in to the 5 but not the 15.
Only for the attacking team. There is no logic to moving a defensive PK to the 5m line, but WR did this some years ago.
Moving in 15 is what happens when there is an infringement while the ball is out of play, so there is logic in OBs suggestion.