I see this as an awesome opportunity to do something a little mind-bending. It's possible that the true patron controls the patron granting spells, or even that a mover and shaker controls an entity that controls an outsider that controls the witch.
While this seems just annoyingly complex at first, you might consider making the patron actually opposed to the personal goals of the PC. This leads to a certain amount of drive to discover the patron's identity, and plenty of buffer to drive adventures, while leaving the details vague enough that you can actually change them as you get a better feel for what would really move your PC.
