If Adam and Eve aren't real, then neither is sin. That simple. No way around it. So why is he death of God's Son necessary again?
Who says Adam and Eve weren't real? In your rush to criticize God for using a writing style you disapprove of, you overlook that He isn't limited by your logical confines. However He chose to create life, and with it man, He held absolute power to instill the soul of His children into the physical shells any way He wished. I know I am no one to tell Him He couldn't do it in any particular way.