I could not do justice to Yaro's poignant prose, but I see things have developed thus:
1. Good and Evil are just made up subjective assessments. Yaro demonstrated the arbitrary nature of such assessments with a few examples. I don't believe you addressed this issue.
Given the arbitray subjective nature of "Evil", it's existence is far from "necessary".
2. Entertaining your idea, that Good and Evil (and God) exist, Yaro states correctly that God could have made a world without Evil (like the Garden of Eden example). But he hasn't. That's not real nice.
3. You say that Good and Evil have to exist otherwise we couldn't make choices. This is nonsense. We could choose between really good things in a world that had no evil. Free will is independent of these Good and Evil concepts you are laboring over. (Once again Yaro emphasises the fact that Good and Evil are subjective and arbitrary).
4. Finally Yaro points out that if God has the ability to know everything that is going to happen (whether he exercises this ability or not: see Crashfrogs post above) then we don't actually have free will, so the whole Good and Evil choice thing is nonsense anyway.
See the last paragraphs of Yaro's post 41. It's pretty clear: you have talked yourself into a deadend.
You have identified why it is difficult to discuss the issue of free will with Christians. They are horrified by the prospect that free will might merely be illusionary. Our entire court system would also collapse without the supposition of free will.
That doesn't mean that it is real.
It is nevertherless fundamental to all of us to belive we have free will.
If things are pre determined, our free will is illusionary. If we are merely complex bags of chemical soup (which I agree), free will is also illusionary. Either way the Christians are sunk: you may think you have a choice, but you actually don't.
Roboto85, Crashfrogs post does not actually help your case at all.