This *should* be pretty clear cut; changing the destination airport should definitely allow you to get a full refund without any penalty. I detest companies that deliberately stall clearly due refunds like this -- any retailer can refund a credit card charge in seconds, with it showing up instantly on most on-line statements.

Personally, since there isn't another DL that you really want them to reroute you on instead, I'd book the other airline before that fare goes away. Likely you have 24 hours with that airline to cancel if needed, so I'd call DL again first thing tomorrow, inform them that the offered substitute flights are not acceptable, and you are choosing to refund the tickets and expect the full amount to be refunded to your card.

If you don't get an encouraging response, consider calling your credit card company -- mine has been great in the (very rare) situations like this, including once where an airline double charged the card, then claimed it would take several weeks to process a refund. The card company took the charge off immediately so I didn't have to pay it, send me paperwork but told me to wait -- when the airline finally did their part the dispute cleared automatically.