AR 3-5-2-IV IMO clearly says that the player who "loses his status as a participant" must adhere to the substitution rules. If he comes inside the nines he would have meet the requirements to reestablish himself as a player as required by 3-5-2-d. He doesn't do that in the AR case play.
In the current case play I don't see any foul if the player who incorrectly left with no substitute entering the game meets the all of the substitution rules including being inside the nines when he returns since he was never a "replaced player".
As Kalle states, the AR is very clear. When a player enters the team area, his status becomes that of a replaced player, regardless if he is actually replaced or not. As a replaced player, he must remain out for one play.
The AR is interpreting Rule, and the interpretation is ever so clear. Sure - it would be nice if the rule language explicitly matched the AR. If it did, though, we wouldn't need the AR, now would we?
This is very easy to remember: Come in, stay in. Go out, stay out.
Or in a related scenario if the coach immediately decided to return a replaced player for a another player who was on the field for the previous play that certainly is legal.
Absolutely incorrect. A replaced player must remain out of the game for one "play." With the rule and the AR, any player that leaves the field (with or without a substitute) must remain out one play. After he is out for one play, he becomes a squad member on the sideline, and may become a substitute.
Now, for those that want to complain, there is a legitimate complaint regarding the term "play," versus "down," as the new helmet rules have brought to the forefront.