Dl often has a rectal-cranial inversion when it comes to having RUC's applied.
I would suggest not making the backup booking. That can only mudiy things,. You can try to call back, may have luck may not. If not resolved, give yourself an extra 30-60 minutes at the airport.
I would be fairly confident DL will fly you, Not that confident in what class or on what flight. If not in F on the ticketed flight demand compensation.