Sorry if it seems like I'm running a rut here, but I want to bring up the idea of perspective again.
Anybody with more than a passing interest in science-fiction is familiar with the idea of time-dialation. Simply put, when you go really fast, time slows down for you. The "twin paradox" is usually used as an analogy: You have two twins, say Joe and Jane. Joe stays on Earth while Jane goes zooming around the galaxy at light speed. When Jane comes back, Joe will be older than than she is, because she has experienced time-dilation and he has not. (I'm sure you've all heard this example a million times.) Let's say Joe ends up being 100 and Jane is still just 10.
Now, by my understanding of Relativity, there is no arbiter between Joe and Jane's clocks. The fact that Joe has experienced the passage of 100 years and Jane has only experienced 10 might tempt us to ask the question "Whose clock is right? Was it really 100 years or was it really only 10?" The answer to that question is "Yes." As much as it offends our sensibilities, it really was 100 years and it also really was 10.
Relativity produces lots of other similar effects. "Is the mass of this brick 1 kg or 10 kg?" "Is this rod one foot long or three?" "Yes" to both questions.
I don't know that it will satisfy anyone else, but I think the answer to free-will vs. predestination is along similar lines. I think both are ultimately different terms for the same thing. When you ask "either or" the answer is "yes," although that answer is counterintuitive. The answers don't contradict each other: It's just a matter of where one is standing when one asks the question.
Someone in the timeline says "I have free will" and he is right. Someone outside says "his future is fixed," and that is also right.
And no, I can't really wrap my mind around that idea myself. But, if we're dealing with eternity and infinity and the nature of being, would you really expect to be able to totally grasp it? Because we do not have access to an "outside" view, the best we can do is resort to metaphors, and they will never be exact.