According to Arminianism, which stresses the free will of man, each man’s salvation is totally up to him; that is, it is determined by whether or not he chooses Christ. According to Calvinism, which stresses the sovereignty of God; each man’s salvation is totally up to God; that is, to whether or not God has chosen him. Calvinists believe that only the elect−those chosen by God−will receive the divine aid and intervention necessary to be saved. Thus, in the Calvinist’s mind, all men are free to choose sin over Christ for themselves, but only those chosen by God are capable of choosing Christ over sin.
Obviously, these are deep spiritual waters that you are requesting us to tread. Throughout Christian history the church has often wrestled with these two seemingly incompatible truths; namely, the free will of man and the sovereignty of God. I readily admit my inability to explain in this short answer to your question something that Christianity’s greatest theologians have been unable to explain. May it suffice you at this point for me to point out that we need only to believe what the Bible says; nowhere are we called upon to understand it all and explain it.
Second, Adam and Eve did not “fall from grace,” but from their perfect created state. Their fall is what necessitated the grace of God coming into this world, in particularly coming into this world in the person of Jesus Christ (John 1:14). For fallen men−the descendants of Adam and Eve−to be reconciled to God and ultimately restored to the state for which God intended and created them, Christ had to come into the world and do for us what we could never do for ourselves. Consequently, as a result of all that Christ has done for our salvation, we can now be saved by God’s grace through our faith in the sufficiency of Christ’s finished work for us (Ephesians 2:8-9).
Finally, you ask the question: “Why would God create man to destroy him and have to start over?” Far from creating man to destroy him, the Bible teaches that God created man for fellowship with Him. To further elaborate on this great biblical truth, the Apostle Paul teaches us in Ephesians 3:2-13 that God’s eternal purpose in all that He has done has always been to secure for His Son an eternal companion, a bride−the church. Herein lies the answer to your question about man’s free will.
When you ask “why” God “didn’t stop” Adam and Eve from disobeying Him, since He knew “what would happen to his creation” once they did, you’re overlooking the fact that God’s overriding of our original parents’ ability to choose for or against Him would have rendered their love valueless to Him. How much would you value someone’s love who was under a spell and couldn’t help but love you? The value of love is found in the fact that we choose to give it. If choice is taken out of it, all value is removed from it.
God did not create puppets or robots, but men. He created us to love Him and to fellowship with Him. Thus, He had to give us the choice to do so; otherwise, our love and fellowship would be completely meaningless to Him. The fall of Adam and Eve in the Garden, as well as the flood in Noah’s day are two of the horrible tragedies that have befallen man because of man’s wrong choices. Likewise, all suffering in the world today is caused by mankind continuing to make the same mistake that our original parents made in Eden, namely, choosing to love ourselves rather than the loving God by whom and for whom we were created.