If indeed End of The World is a Realm of Darkness world, then why didn't Sora pass by an in-between world? Kingdom Hearts blocked the Realm of Darkness, so it' safe to assume that End of The World is an in-between world leading to the Realm of Darkness. As for your many questions, I can answer the nobody one. Sora's abilities were hindered and while he was sleeping, his restoration was quite slow. Ansem, or DiZ, theorized that because he had lost a part of himself, he was recovering at a much slower rate. Kairi was reunited with her nobody much later in the game because Kairi completing herself ..well...served no purpose to Ansem or anyone else in the game, for that matter. Sora, on the other hand, needed all his strength to achieve Ansem's revenge on the Organization.
Then how come when that large keyhole of Hollow Bastion was opened by the princesses, darkness sudden poured out of the keyhole (which I believe actually came from the Realm of Darkness and actually not Kingdom Hearts)? Also, Traverse Town is indeed an in-between world, as confrimed by the Secret Ansem Reports. I don't know for sure about End of The World, but it's made from pieces and the leftovers of destroyed worlds. If End of The World just so happens to be near Kingdom Hears, does this mean that Kingdom Hearts also serves as a boundary from the in-between and darkness realms?
Oh my god... This report is answering all the wrong questions. Simce when has the pods become such a plot hole? The real questions come from the frist installment. Why does Hollow Bastion contain the Door to Kingdom Hearts? Who was that voice in the beginning? Where exactly is Kingdom Hearts located? The report also forgot to include Traverse Town and End of The World. Aren't they also in-between worlds? It was as if KH1 was knocked out of canon.