Ah. The presence of an omniscient, omnipotent being is important to the proof, but it is not the only element in that proof (the other elements are taken from observation of the universe, and are less controversial).

Consider; if an omnipotent, omniscient being exists, then it must take one of three stances with regard to humanity. It must either support the existence of humanity, or it must be neutral towards humanity, or it must support the non-existence of humanity. Since the being is omnipotent, if God wanted to wipe out humanity, God could (one or two well-placed asteroids a couple of million years back would have done it easily). Thus, I conclude that God is either in support of, or neutral towards humanity.

Now I also observe the universe around me, looking for traces of maliciousness in the laws of physics. So far, I have not found any. This implies that God is not into casual, petty cruelty without reason. It seems therefore likely that God is, at the very least, not evil.

The complete cessation of an intelligence would seem to be a great evil. Therefore, I postulate that there is a very strong probability that God has put some measures in place to prevent this. The measure most likely is some sort of afterlife; somewhere that a person can continue to survive, but not communicate back to those they leave behind.

Of course, this argument does not say that an afterlife is certain, given the existence of God, merely that it seems likely.

As to free will; here, I note that humans are demonstrably capable of the sort of casual cruelty that is absent from the laws of nature. Moreover, humans are capable of opposing each other. This strongly implies that at least some humans are capable of opposing what God wants. (This does not necessarily imply that said opposition has any chance of long-term success). This, in turn, seems to imply that humans do have some capacity to decide for themselves; hence, free will.