Prospero maneuvers the whole scene in The Tempest. How far is his Machiavellian manipulation justified? He himself brings his ill-fate.
He gains Milan by sorcery(a malpractice) not by pure discretion.
First, you should know that Milan is Prospero's dukedom. He is the "right Duke of Milan." Prospero made his brother, Antonio, the manager of his state, but Antonio became greedy and power hungry and, with the help of Alonso, King of Naples, had Prospero declared "incapable" of ruling and set out to sea with his small daughter Miranda to, Antonio hoped, die. All of this is related in more detail in Act I, scene ii. It is very important to understand this situation, since the restoration of Prospero to his rightful place and the forgiveness of Antonio and Alonso is a huge part of the play.
Now, for your question. Machiavellian might be a term better used to describe Antonio's actions, since he was a master behind the scenes manipulator of political events to his own advantage before the events of the play, and he continues this trend when he persuades Sebastian to follow his lead and knock off Alonso. Actually, Propsero's problem as a Duke seems to have been his lack of interest in politics and actually owning the responsibility of ruling his own dukedom, hence his having Antonio to manage it for him. In this way, Prospero, as you say could be seen to "bring his ill-fate," being cast out and stranded on the island where he begins the play.
At the end of the play, Prospero forgives Antonio and is restored to his rightful place as Duke of Milan. He leaves behind his tools of magic (his robe and staff) and says that he will "retire" at Milan, "where/Every third thought shall be [his] grave." So, it is definitely worth wondering if he'll be able to return to his dukedom and rule it himself this time, but there is no evidence in the play that he is headed to an ill-fate of any kind, nor that he gained Milan by sorcery, since he was the correct and rightful Duke to begin with.