When you file a claim, you have to supply the Diners Club statement showing the car rental, and you also have to supply the car rental receipt. So they can easily see whether you charged the full amount. Because of Diners' "dual account number system", they may not immediately see whether you receipt is showing the corect card if it's showing a card at all, but if it doesn't show a card at all, I suspect that could send up a red flag. But they might also reject it if the Pricline entry on the statment doesn't have the same date as the rental, or doesn't even mention the rental date.

And you might not know any of this until maybe weeks or months into the claim, because the claim process is kinda slow.

So I would assume that it won't work to be on the safe side.

How much you might want to worry about this might depend on where you're renting a car relative to where your normal car insurance (assuming you have that!) covers rentals. The typical US car insurance will cover rentals at least in the US, but typically not far overseas (maybe adjancet sections of bordering countries, maybe not). If your Diners coverage doesn't work in the US, you're only out a deductible (and the blemish on your insurance record). If your Diners coverage doesn't work somewhere where you have no other coverage, that could be much more serious (as the rental car company would hold you liable for not only the cost of the repairs whichever way they want to do the repairs, but also for "loss of fleet utilitzation" costs!).

Thus if you don't have backup coverage that works no matter how you pay, I would suggest you use the Priceline savings to buy the CDW option! (And if Priceline doesn't save you enough to allow you to buy the CDW option and still save net, then was it worth pursuing in the first place?)