Illegal Substitution vs Participation

[mcrowder]

This is predominantly an NCAA question, but if Fed rules are similar or identical, Fed opinions will be appreciated.

NCAA RULE 3-5-2-b: No player, in excess of 11, shall leave the field of play or an end zone while the ball is in play (S22). PENALTY - five yards from the previous spot.

Note: S22 is Substitution Infraction.

NCAA RULE 9-1-4-a: No substitute ... may interfere in any way with the ball or a player while the ball is in play.
NCAA RULE 9-1-4-b: Participation by 12 or more players is illegal participation.

One interesting note in 9-1-4-1: PENALTY (for illegal participation): 15 yards from the basic spot. The referee may enforce any penalty he considers equitable, INCLUDING awarding a score.

I assume we would not award a score unless someone came off the sideline in the middle of a play and made a tackle... but it does raise the interesting question about whether we can enforce 15 at the END of a run, instead of only from the initial Line of Scrimmage.

Here's the MAIN question (sorry for the long prelim). On a play where the defense has 12 on the field, and runs one off, but not in time (player 12 does not "participate"), every official I've ever worked with either A) blew it dead before the play started, and walked off 5 for Ill. Sub., or B) let the play run and flagged it and called it illegal participation. In a game last night, we discussed and determined that this was simply I.S. - and walked off 5. Is this in contradiction to common practice? Were we wrong? And what of the thought that if 12 players participate meaningfully, can (or should) we march off 15 from the end of the run?