I think your concerns are valid and I would restate the fact that you shipped the item to him per his instructions & note he has to abide by the rules set forth by the PO. He simply has to accept the fact there are time constraints imposed by the USPS.
I would wait the full 30 days (if necessary) then file the claim & find out the exact procedure, specifically, what would happen if you filed the claim & the package was delivered? Could you get it back, would USPS be responsible for it, etc.? Not sure of USPS policy once a claim has been filed, so it's in your best interest to get all the details. I would not let the buyer bully me into making a rash decision & if he threatens a neg. then so be it.
As for documentation, print out copies of all emails & any other correspondence you have with him or anything related to this transaction, including names & #'s of people you talk to at the PO.
As for shipping, if he does receive it & simply wants his money back, I'd do whatever it takes to get my gear back. It's a small price to pay compared to the DAC itself. Hope it works out.