Marcello was paying a fortune to some of the best lawyers that money could buy to keep him from being departed again. From my understanding, Robert Kennedy was so bent on deporting him that not all of the right proedures were taken the first time lawfully. As to why he put down Guatemala on his birth certificate, I think it might have more to do with that he never like Tunisia, or like LaLouisiane has posted above, he wanted to stay in the western hemisphere.