Ugh. This is awful--there is no excuse for the Red Coat taking back any part of an offer they confirmed with you. I would start with the DL customer care number, 404-773-0305. They have special agents for situations where the customer has been wronged like this. Unfortunately, I don't know if they're open on weekends. If that doesn't work, you can try calling DL and asking for a supervisor. They should see the notes in your record and you might get the D1 seat that way.
I don't know if either of those will work--hopefully others will have ideas too. Please report back if you can. Good luck.