it is impossible for both him being omniscient and free will to exist
Ah, but how would he "know" the future? God is a black box, in that aspect. Take the stonelayer from xkcd, he knows all there is to know in the universe, but he has to model it first to see it happen. He is omnipotent and omniscient, and yet he has to do the whole calculation before he knows the answer. Now, for the people in that model or calculation, it's never clear if they're real or just the model. So that's us again. So yeah, God might not know where the universe ends up when he started it, and we don't know which version we are.
Omniscience != Determinism.
He would know the future because he is omniscient. omniscience means he knows everything, all the time, and since the future is part of everything, he would know the future without having to calculate it.
If you did have to calculate it, you wouldn't be omniscient.
Take the stonelayer, he isn't omniscient, he merely knows everything that is happening in the present, and given enough time, can model the future.
That's incredibly evil.
Or he could let us live without knowledge and like animals. I don't know which is evil.
He punished humans with 4000 (according to the bible) years of original sin and kicked us out of the garden of eden, for something that he knew would happen and caused to happen by his inaction.
I can disprove omniscience in one simple step: Heisenberg Uncertainty Principle
I thought that only applied in an open system, like say, every single point in our universe, but were you to exist outside of the universe, you could treat the universe as a closed system.
God is magic, so for the most part the rules of the universe don't apply to him. Like the second law of thermodynamics which he violated or the law of conservation of energy (making a burning bush which didn't get consumed in a fire), or the law of conservation of mass (making things out of nothing, and a couple of the miracles jesus did).