An interesting connection you have made! And a good question, I would say. Please understand that my answer is coming from a Christian, not a Muslim perspective. Therefore, the reasoning may not be the same that you would apply to the question–but you asked me!
First of all, you are quoting from the Koran. I am convinced, and with very good reason, that the Koran is not inspired by God. It tells us that Jesus was not crucified, but he was. It tells us that Jesus is not God, but he is. I have an entire lesson I can share with you if you like which, I believe, shows conclusively that the Koran is not inspired. In fact, here is a link:
Apologetics and Islam
That being true, the fact that the Koran says Adam was forgiven is a moot point. It tells me nothing about whether or not God has forgiven Adam for his sin. Perhaps he has, but the Koran is NOT a source of inspired truth. That is my strong belief.
Then you say that Jesus died for “that sin.” I am not sure what passage of Scripture in the Bible you are referring to here. There is no Bible passage to specifically says that Jesus died for the sin of Adam. Now, it is true that Jesus died for the sins of the whole world (prove by 1st John 2:2. For this reason, one can say that Jesus died for the sin of Adam. However, this substitutionary sacrifice only applies to those who put their faith in him. The forgiveness of sins from God–his grace–is given to those who put their faith in Jesus Christ (Romans 1:17, Romans 4:1-3 and many other Bible passages). Of course, Adam was not able to believe in Jesus, but the passages I quote above show that for those who have never heard of Jesus, they, too, are saved by faith.
My conclusion is that the evidence you provide definitely does not prove that Jesus’ sacrifice was meaningless. This is true, both because you use an uninspired quote from a false prophet to make your point, and because the argument uses false reasoning related to how the blood of Jesus applies to the sin of the unrepentant, such as Adam.
By the way, I am not certain that Adam did not repent. Honestly, I am not sure if he will be in heaven. But that is beside the point of my argument above.
John Oakes