Unless I am mistaken, the agreed upon PWA has not been assumed as of yet. Rather, pursuant to the letter of agreement, Delta has agreed to assume the PWA, as modified, as part of any plan of reorganization filed with the Court. Thus, until the POR is accepted and confirmed, the PWA has not been assumed by DL.
Therefore, it is possible that the PWA may be modified/amended before any plan is accepted and confirmed. Of course, it appears that DL ALPA will be reluctant to modify the PWA in a way that will allow US to succeed in a DL/US merge. However, ALPA's reluctance does not prevent an 1113 motion. A decision under 1113 would not "overturn" a previous decision of the court, as this PWA was a result of a settlement (letter of agreement) and order under 363, not 1113.
Of course, your 1st question is probably: "DL doesn't want a DL/US merger so why would DL ever think of attempting an 1113 motion that would give US a fighting chance of succeeding in a DL/US merge?"
Well, it is not always necessary for the debtor (DL) to file the 1113 motion. In fact, the creditors' committee has standing to seek rejection of a collective bargaining agreement when rejection of the agreement is necessary for proper reorganization and the debtor isn't acting. This is a difficult standard to meet; but if the creditors' committee wants a DL/US merger, I think it may be able to find a way (educated speculation that is likely moot).