How did Xemnas know who Roxas was even if he had been keeping tabs on Sora? I mean, Sora looks nothing like Roxas, I don't see the resemblance at all. Roxas did look like Ventus but Xemnas shouldn't know of the connection between Sora and Ventus. Also, how did Xemnas even know where Roxas would be? It looks so deliberate that he had to know for sure so is twilight town, the place where all nobodies start out?

What about those big letters 'SORA' that appeared when he gave him a name?

Since Xemnas is the controller of Nothingness and Nobody's are made of Nothingness, I'm sure Xemnas has some sort of power which allows him to allerted to the presence of Nobodies. He then would have known of his connection to Sora by somehow knowing the connection between Sora and Ven. (I think it's pretty likely he knew, either before or by seeing Roxas. He difinetly knew, when he saw Xion, since he and Xigbar both see her as Ven). Then he did that whole name change thing changing Sora to Roxas. He gave Roxas the cloak, waited awhile and Roxas joined the Organization.

 

That question has been asked and no. Xemnas and the rest of the first 6 members of the Organization were confirmed to be 'born' in Radiant Garden. It's not exactly known why Roxas went to Twilight Town, but it's thought to have something to do with Ven or another force that drew Roxas to Twilight Town.

Well, Nobodies are usually born in places between the light and darkness, such as twilight town. It is weird that Xemnas new he was Sora, I was thinking the same thing, but maybe he was able to sense Roxas being connected to Sora. And nobodies are able to reach into people's hearts and see memories, maybe he was able to see hidden memories that he was Sora.

What about those big letters 'SORA' that appeared when he gave him a name?

 

Wow, you just made my day.

 

Xemnas was at Hollow Bastion during the events of KH, as evidenced by the boss fight in Final Mix. He could have watched Sora stab himself from the shadows.

Okay seriously, is everybody missing

?