A1 is injured and treated on the field. A subsequently requests a time out. During the next down, A1 runs 20 yards for the go ahead TD. Following the score, B requests a TO so the coach may discuss A1's participation with the referee. Ruling?

This is an illegal participation foul. Enforce 15 yards from PS, and A1 must sit one play.

Not questioning your ruling, just questioning myself....

Does he have to sit for one play after commiting the Illegal Participation foul?

Yes, because he has not yet been "replaced for at least one down," which is how the rule reads.

Taking this one step further, the coach who requested the time out to alert officials of the participation, should not be charged for the time out, should he? Also, the ball should be spotted at B's 35 yard line (since it was a 20 yard TD run) and replay the down. And since A1 illegally particiapated in the previous play, does he have to sit out the next play even though his team was penalized for his participation in the previous play?

REPLY: Let's take them one at a time...
1. The coach will initially be charged a time out for his coach-referee conference. Once the referee agrees that they misapplied a rule, that time out will be rescinded and it will be deemed an official's time out.
2. You're correct. The foul is illegal participation simultaneous with the snap. Penalize 15 yards from the previous spot and replay the down. That's a key point, because...
3. the injured player will still have to sit out the next play because it's a replay of the down in which he fouled.

And remember to start the clock on the snap.

Canadian Ruling
 

Ooops. Play on.