Here is my own analogy;
Let's say your 22 year old son is a motorcycle stunt driver. He wants to jump over 25 cars like Evil Knievel used to do. You know in your heart that your son is not capable of performing this dangerous stunt and repeatedly beg him not to attempt it. But he is a grown adult who can make his own decisions and you cannot stop him.
So, what do you do? You know in advance he is going to attempt this stunt, and you are pretty certain he is going to crash. So, before he actually makes the jump, you make sure there is a team of emergency medical personel to help him when he crashes. When he crashes, they can immediately tend to him and hopefully prevent him from dying.
This is how I think it was, God knew we would sin, and he planned ahead and had a Savior ready to save us when we did. But he never intended or planned for us to sin in any way, just as you did not plan or intend for your son to perform this dangerous stunt.
sorry, but you are portraying a God that can't get what he wants. You have a God that wanted a perfect world but man overpowered and sinned anyway. I agree that God allows people to sin as the first part of your analogy showed, but you error comes at the end to say that people sinning wasn't part of God's plan. Jesus is not an afterthought. I understand your difficulty with it. It's hard. It's called the problem of evil. We just don't understand it all, but we can't ignore it. The Bible clearly states that God uses sin to accomplish his will.