Interesting point about Nineveh. And especially considering Nineveh has a purpose in the future as well:
Mat 12:41 The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here.
I am thinking the primary purpose of the prophets was/is to warn people to make them repent.
People like to argue for free will, but Paul seems to preach against it in Romans 9.
But, could we have free will and there be predestination? Because God Was, and Is and Is To Come, He sees the beginning to the end, so He knows what we will do with our free will, and what we have done with our free will. God hardened Pharaoh's heart, but Pharaoh also hardened his own heart - God would've known that Pharaoh would harden his own heart, so maybe God helped him along with it?
Then again, it's not about us. It is about God and His purposes, and His plans and like it's written:
Rom 9:18 Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth.
That is pretty straight forward.