I'll add a couple items and offer a suggestion or two.
First, a disclaimer: I am no fan of Capital One. I believe they are a deeply dishonest company that uses extremely deceptive marketing techniques to take advantage of confused consumers. I would never, ever recommend doing business with any company that is so deceptive.
Second: Capital One's program is NOT a conventional frequent flyer (loyalty) program (one can argue that it's not one at all). The so-called "miles" you earn with Capital One are not "miles" at all in the sense that the term is used with other airlines' programs. All you are earning with them are "pennies" - literally, pennies. You get a small credit (which they call "miles") which you can then save up to use to buy airline tickets at the market rate. This is generally a terrible deal (because you are paying the retail price for a ticket, just with the discount of using your saved-up pennies). The great benefit of using real frequent flyer miles in other programs (which have historically not been directly linked to the cash price of a flight) is that you can get great value by picking and choosing wisely.
Capital One basically gives you a tiny rebate on your spending, in a form of cash, which you can then use to trade in for cash-back. You do not transfer your Capital One points to an airline (with more traditional programs from Chase, Citi and American Express, you do exactly that: tranfer your "bank" points to an individual airline's program, then they become indistinguishable from points earned and used directly in the airline's program).
(Caveat: That's the way Capital One has always worked with their so-called "miles". They recently introduced a more conventional program where you actually CAN transfer their points to some airlines, though at a relatively bad rate...I'm assuming the OP has been using the longstanding Capital One system where they earn "miles" which are actually pennies, and then they just spend them like cash to buy a ticket at retail rates.)
So in this case, it seems the airline and Capital One are quite separate. Capital One probably isn't going to do much to help (and their CS agent certainly didn't sound anything like helpful). I would stop calling Capital One and focus on the other party: Delta.
The OP needs to contact the airline (Delta) and deal with them. Expect a long hold time (there's a lot of that going around). You bought a ticket from Delta. Yes, you used a weird form of currency (Capital One Pennies) but you still paid them. So ask for (do not demand) a refund. Ask nicely, since it doesn't sound like they are required to give you one. You may (probably will) have to settle for a voucher or credit for a future flight. But it doesn't hurt to ask nicely.
I'd wait until 1-2 weeks before your departure date before trying them. They are slammed, and scrambling to cancel flights each day, trying to deal with the most immediate cancelations. Good luck and bring patience.