I would put it this way in the context of our discussion: man having free will does not necessarily mean God cannot have a plan.
Your initial point, however, was that God's omniscience/omnipotence logically negated man's having free will. I don't believe you've proved this statement.
Let’s say God knows that you will die at the age of 87 in a terrible car accident. He creates the world anyway, even though he knows that.
What age are you going to die at? How are you going to die?
What difference do any of your choices make in the outcome?
I think part of the difficulty of people getting this is that they think of God in time and not eternity.