Dead ball fouls prior to the snap

[bigwes68]

I think you got it right, per casebook play 3.7.3 Situation A:

"B11 mistakenly believes he is his team's 12th player and leaves the field before the snap (a) on his opponent's sideline, or (b) on his sideline and enters his team box. B11 then discovers his error and returns to field on his team's side of the neutral zone before the snap. RULING: In (a) B11 has committed illegal substitution by leaving on the opponent's sideline. (3-7-2) In (b), once he left the field, he was no longer a player, and by coming back onto the field, is an illegal substitute. (3-7-3) If done intentionally to gain an advantage, it is illegal participation foul or it could be an unsportsmanlike foul. If B11 returns to the field after the snap in either (a) or (b) and participates, it is a live-ball foul for illegal participation. (8-5-1f; 9-6-4 a and d)"

Had the same thing happen in a game I was working early in the season, except it was an A player that went off the field and came back on.