1. Little Hawk: Atheism is not the assertion that there is "nothing beyond human understanding." It could be, for example, the belief that it is extremely improbable that the universe was created with humanity in mind, by a Supreme Being who places humanity before all else and sounds very much like a human on steroids. Or an atheist might feel that the idea of a "supreme intelligence" unduly exalts a trait of which humans are already unduly proud. Or that it is extremely doubtful that an "all-wise Creator" would hold some of the astoundingly unwise views often attributed to Him. Or that a benevolent Creator would create not only the wonderful things that exist in the universe, but also the many horrible ones as well. Or that He would endorse the cruel things often done in His name.
2. I sing all kinds of songs that stand for things I don't believe in. I sing religious songs, mostly, when the feeling in the song is one I share, even if I don't share the theology. Songs that give comfort in adversity have something in them that gives comfort, and does not require that there is an all-powerful being out there that takes a personal interest in protecting one's well being.
3. And "Amazing Grace" is by no means the best there is.