> In a completely deterministic universe, I believe you would (theoretically) be able to perfectly predict what any entity would do under any given circumstances. This is called Isaac Newton's "Clockwork Universe" theory, and it does negate the presence of free will.

This is indeed what Newton believed, and it would indeed disprove free will, but it is incorrect. As I said, dynamic systems can, and very often do, propagate errors exponentially. What this means is that any error in the knowledge of the initial conditions, no matter how small, will fairly quickly grow to affect the entire system. This is one of the more important discoveries of chaos theory. It is what is more commonly known as the butterfly effect.

This means that to predict the outcome of a system over arbitarily long timescales requires infinite knowledge of the initial conditions, and this is impossible to acquire, if for no other reason than that the universe is finite, and you could not fit the information inside it. Thus, even an entirely Newtonian, deterministic universe is unpredictable.