I know we are suppose to just tell things we don't understand but I'd like to answer two of your questions.
(SPOILER)It represents Riku, its Japanese name is "Passing Memories" and due to it only being obtained in Castle Oblivion after the Riku fight, its suggested to be a representation of Sora's connection to Riku. The teeth of the key is apparently fashioned to look like the kanji for darkness while the Oathkeeper's is the kanji for light.
(SPOILER)When Sora used the Dark Keyblade on himself in Castle Oblivion, it affected both his own heart and the heart of Kairi who was contained within Sora's body. Kairi never got transformed by a Heartless but her heart underwent the process. It didn't create a Heartless because she is one of the Princesses of Hearts but it allowed the creation of Nobody... despite not making much sense since Nobodies are the discarded remains of a Heartless' former body. Yet then again, its stated often that she is not normal for even a Nobody.dont under stand how namine is kairi's nobodie if kairi never became a heartless.