Substitution question

[ChickenOfNC]

Possibly a two part question:

I realize under 3-7-3 that it is illegal for a substitute to become a player and then withdraw and vice versa during the same dead ball period. However, is it legal for the 11th player, who wrongly thinks he's the 12th player, to leave the field, realize his mistake, and then return?

Part 2 (If the above situation is legal): If this player is an offensive player, and before leaving the field had established himself between the 9 yard marks after the RFP, must he re-establish himself between the 9's when he returns to the field?

(All of this assuming an absence of intent to deceive, etc.)