In theory only the package holiday passengers with ATOL protection would be entitled to be flown home, so they may not need an A321 if they decide to leave the flight only pax behind. That's going to become a very volatile situation at the airport if that's what's about to happen.
I would think we should be hearing something from the CAA within the next 2 hours before those aircraft land in Ibiza and the pax there are told what's happening.