NCAA Unreported sub
 

8.3.3.5.3 The unreported player has not made a play, and a pitch has been thrown.
EFFECT: The unreported player is declared officially in the game, and all play will stand.

Top 2nd inning, DC places a unreported sub in F9. After a pitch, and when they are then on offense, is it correct that that player may legally come to bat for the original F9?

Need to look at 8.3.3.5 first:

If the unreported/misreported player is reported by the offense before the defensive coach in violation informs the umpire, and:

Then you can go to 8.3.3.5.3

8.3.3.5.3 The unreported player has not made a play, and a pitch has been thrown.
EFFECT: The unreported player is declared officially in the game, and all play will stand.

The effect only occurs when the plate umpire is notified of the substitution. Just because they slide by one pitch without the OC figuring it out doesn't put them in the game.

If the umpire is not notified of the sub on defense, then the USub bats and gets on - you better bet your house the DC is coming to get their out.

8.3.3.5 is the part of the rule where the unreported sub is playing defense, but is reported to the umpire by the offense. If the sub has still not reported when she bats for the original F9, we are now under the jurisdiction of rule 8.3.3.3.

The sub is not legal until properly reported by her own team, or "discovered" by the opponent. The NCAA rule book has a handy Improper Player Chart in the back of the book for the correct procedure for each of the 28 ways a player can improperly participate.

She doesn't even have to get on base, just take a pitch as an unreported batter. 8.3.3.3.1 - A pitch has been thrown but the unreported/misreported player is still at bat. EFFECT: The unreported player is called out and is declared officially in the game. All other play is legal.

Yes, she can legally come to bat, because she is legally in the game, per the rule you quoted, BUT her status as unreported remains until/unless she is reported by her team, or protested as unreported by the opposing team.

The point here is that she isn't "illegal" if she could legally occupy that position in the game; but she is still "unreported".

Except, if I'm the opposing coach, I'm NOT addressing this until it benefits me most; sure, I can as soon as she takes a pitch, per the rule you quote. But this is my ace trump card, and if she makes an out I let it ride; because she is STILL unreported. When she makes a play on defense, or does something positive on offense, THEN I pull my trump card, and get the play nullified. I only get to use the trump card once; and I only lose out if her team realizes they never reported her and nullify my trump before I get to use it. But is she hasn't done anything to hurt me, why play that card?