The problems are two fold.
If we have left Guantanamo, (although the time is short for us to logistically do that) and the Cubans were to march in, no US law is going to get the Cubans to leave.
The pardon of a mass murderer or multiples of them? Totally legal and irreversible. He is already mass pardoning armed drug dealers. Are Mumia Abu-Jamal or OJ next?
If he orders the military to abandon Gitmo with no notice, that might be irreversible. But moving people and their families out of Gitmo without the press finding out might be tough. If the press finds out about it, lawyers could get a court order preventing it by allowing civilian oversight to take control of the base. Cuba only gets it back if every single person has left, so even if all the military personnel follow Ozero’s orders to abandon it the presence of civilians on the base would block repossession.