From the days when AS and DL codeshared, I remember it is easy to give a ticket to another carrier, but impossible to take them away. I don't think OP understood the risks and I think the agent did the best they could, but things unfortunately did not work out. Communication is very important when dealing with multiple parties. When I used to have IRROPs on AS, I would make all the changes post security with the gate agents in control of the flights. My first step would be to talk to an agent from the airline that holds the ticket (DL in OP's case) and ask if they would be willing to sign the ticket over to another carrier. Then, I would go talk to the agent in control of the flight I wanted to be on (the UA flight in OP's case) and tell them what I was doing, and ask them if I could hold a seat. I would then go back to the agent for the airline holding the ticket (DL in OPs case) that I previously talked to and ask them very nicely to sign the ticket over. Most recently, I had a redcoat (supervisor) call to another airline to see if they had space available on an alternate flight during an oversold flight. Point is, you need to ensure that you have all your ducks in a row before you do something like this, because there are no takebacks.

Also, I doubt someone else jumping on the CVG itinerary would have mattered (DL would have oversold it if they had to) - it was more likely they could not get the ticket back from UA.