The idea that free will and sin create the conditions for the ‘building of character’ is a pretty strong argument. Yet it doesn’t answer the question of why evil has to exist. Like someone said earlier, free will could still exist without evil or sin, you’d have a series of choices that led to different results, which could even be destructive (which is not necessarily sinful - creation implies destruction or reformation)...but the absence of a sinful nature would allow us to avoid ‘sinful’ reactions and feelings related to our decisions. The whole idea of sin seems self-referential...’it exists so that God can get rid of it in X thousand years’. Will it happen again? Will we become robots without free-will in heaven?
I did not accuse God of being evil. He must be imperfect, if he is to be a non-contradictory Christian God, and if perfection implies the intolerance of evil. I’m willing to entertain the idea that God’s moral calculus is way beyond anything we can comprehend, but I don’t like people telling me the secret to it all is as simple as basic arithmetic...because when you do the actual math, it doesn’t compute.
I don’t think anything about God is simple, except for the core which is love. And your view that God created something to get rid of it in a thousand years is in a way correct; he created the law to demonstrate to man and angel that even a simple set of rules cannot provide salvation (man cannot save himself through perfection) but must look to God and enter into a relationship with him. The relationship is the end, the goal, the desired result. A people for God.
As far as not having a sin nature it was tried. Man did not have a sin nature until he chose freely to sin, but turned from God and listened to evil and obeyed evil, not God.
The idea that God is imperfect is not Biblical; it essentially makes the Bible a lie, which makes God not truthful as well! To view God as imperfect puts an awful strain on the universe . . . it’s a complex work to have errors in it . . . I think the magnificence of it argues for perfection and not the reverse.
I really think that Leibniz got short shrift. Voltaire made terrible fun of his ideas in a play, but the idea that God created a world with the absolute minimum of evil required to achieve his goal of a free people in relationship with him is a powerful one for me.