To me, there's really just one important factor that is working in favor of Prospero.  That is the fact that he can do magic and can control Ariel.  It is this ability that has put him in the driver's seat by the time this scene begins.

I guess you could say that there are three things that are working in his favor because of his ability to do magic.

You could say that it is in his favor that the king and Gonzalo are sad and are probably in a mood to repent.  You can say that it is in his favor that Ferdinand is totally in love with Miranda.  And I suppose you can say that it is in his favor that Antonio and Sebastian are plotting against the king.

But all of this has come about by his magic, so I think his magic is his real advantage.