If I recall, the technical order when playing a card is (1) reveal, (2) make check, (3) place in card pool if it passes. If so, then does this mean that the action side of HPDS does not get progressive difficulty from the card you are trying to play? i.e.:

I attempt to play Diving Wind Kick Throw at 9 difficulty and check a 5. Then, I respond with HPDS at 5 difficulty (since DWKT isn't technicaly in my pool yet) and pass it with a 5, making my DWKT check 7 so that I only have to commit 2 foundations.

Something like that? Seems like a good thing to clarify...

correct on all counts.

similarly, with Tag Along vs Akuma::, the card you attempt to play doesn't add difficulty to TA

I didn't know about this.

Perhaps that action side is playable, after all.