I believe that the story was literal. The reason is because of the fulfilled prophecies. If God is able to predict the future, which is a very difficult thing to do, it gives me confidence that His account of the past is also accurate. If you are interested, I shared a few of my thoughts on this topic in a recent YouTube vlog entitled "Is the Bible Reliable?".
Adam and Eve ate fruit, but the real issue was that they chose their own way over God's way. This action opened a way of life that resulted in untold evil. I suppose in this sense, all the evil could be attributed back to that first act. But I think it is more accurate to view the way of life, continued through the years, as the real culprit.
As far as mercy in the Garden of Eden, God told Adam and Eve that in the day they ate of the tree of the knowledge of good and evil, they would surely die. Yet they did not die. However, something did die, because God clothed them with skins, and then they were allowed to live on, at least for a time. This experience was a symbol of Jesus. Paul wrote, "All of you who were baptized into Christ have clothed yourselves with Christ" (Gal. 3:27, NASB). When we are baptized, we are united with Christ, just as Adam and Eve were clothed with the animals in Eden which were sacrificed. And then He grants us eternal life, which is even better than Adam and Eve being allowed to live on for a time. And one might even say that Adam and Eve were symbolically united with the animals in Eden. After all, they sure looked like them after being clothed with their skins. Thus, in a symbolic sense, Adam and Eve did die on the day they ate from the Tree of the Knowledge of Good and Evil. Likewise, when we are united with Christ, His death counts for us because He considers us part of Himself. This union is so close that He considers our sins as His own, thus His death can justly pay for our guilt.
As mentioned already, this union happens because of baptism. Baptism is a symbol of death and resurrection. In other words, believers respond to their sinful desires as a dead person would, and they respond to Christ's desires as very much alive. This way of life undoes the problematic lifestyle introduced by Adam and Eve, and restores people to a path that will result in eternal happiness.