Wow, I like that. Sadly, I'm never fast enough on my feet to think of these things in real-time. Definitely, the GA should have gone on-board and apologized to the lowest-ranking upgrade that there's been a mistake and the upgrade was not properly issued. Yeah, you'll end up with someone who's bummed about losing out on their upgrade but, in this case, I think it's more important the Delta honor the deal they cut with the OP. To avoid the entire mess, the original Red Coat should have phoned the other GA and advised him/her to stop processing upgrades until the OP arrived.

To the OP, yes, a note to DL is in order. As others have advised, just stick to the facts of the VDB. I would point out that they should have honored the complete VDB agreement.