as per the topic, what point will a card be classified as being in the card pool. if my opponent modifies a control check and I have a card in hand that doesn't match the card I'm attempting to play (that either self clears or I can clear) can I respond with that card?

ex.

I'm playing a good/order/water attack as the first card played into my card pool, opponent hacks my check, I respond with an action card that doesn't match the attacks resources (but matches the character) with text along the lines of. R: before failing a control check that was modified by your opponent, that check is successful intead, draw a card and discard this card from your cardpool.

A card enters the card pool only after you have successfully passed the control check for the card, and resolved all costs associated with playing the card.

So yes, what you're trying to do will work fine (e.g. responding with set 12's anti-CC-hax action cards off symbol) as long as you're doing it into a clear card pool or at least have a chain through the action card.