i think it takes place further on in the future like when they are adults and i think the note in mickyes bottle is about the war to come.
i think that the three people in those uniforms are sora riku and bhk cause one of them has two keyblades check it out
and there is a blade that the one on the right gets that has the heartless symbol
also it could be a graveyard but that could mean that sora riku and bhk died and there keyblades where the greatest
we will find out when we play the game about what happens to the keyblade during the game