Now that's most interesting. I kept flipping back and forth between EWTN and FOX. During the Ave Maria, I was watching only EWTN. Indeed, she did have her back to the audience. Earlier in the afternoon, Arroyo's cameraman showed 'KellyClarkson' on stage, hair down, dressed in casual attire. During the Ave Maria, she had her hair in a bun at the back of her head and was wearing a dress that covered all the essential parts. The Holy Father politely applauded but his expression was nonplused (sp?). She certainly did a beautiful job with this hymn - in both English and Latin, though the Latin was a tad hesitant. Since I don't follow American Idol, I have no way of determining if that voice belonged to Kelly or someone else. Arroyo did mention that she specifically requested to sing it for the pope.
Kelly Clarkson did sing.