Wilder-- Yevon did not die when you killed Sin. He can exist without it, which is how he keeps living even though Sin is killed by the final summoning over and over.
The little kid is just a fayth(bahamut's, actually) that was deemed as a messanger boy. In ffx-2
Spoiler: he shows up to yuna several times, like at right before the perfect ending, to tell her that they'll think of a way to bring tidus back
Dream Zanarkand doesn't have a physical location as far as I'm concerned. It's a *dream*.
Most likely Jecht modeled his Sin after what he remembered, because it's what he remembered. Making it a replica of a memory, not the thing itself. Because the thing itself doesn't exist in our dementions-- it's out there in the fourth or fifth one I guess
If Dream Zanarkand was inside Sin, the party would've found all of Dream Zanarkand-- the other blitzball teams, the families, the stupid announcer that was going to run away from home before Jecht disappeared.
The reason you have to get inside Sin is because its outer armor is too strong to be defeated from without. That's why all the ordinary tactics of warfare-- chocobo knights, machina, etc appear to just bounce off of it.
the ball of water that sin used to destroy the first zanarkand was just . . . water. . .
What you really should be wondering about is first off: how did jecht become the only one that was picked up by Sin when it made a random trip to dream zanarkand ten years ago? didn't anyone else go with him? (it makes it sound like jecht is the real main character, and this was his destiny: not his story, but just his destiny
), then how long was Auron in Dream Zanarkand, and how did he convince tidus and his mom that he was a friend of Jecht's (well, she was in considerable distress, so I'm sure she would've accepted anyone that said that and was friendly enough)
about the songs: Otherworld was played when the first bonds with the Dream Zanarkand began to break, and when the last ones were broken. IE, when Sin showed up. All it was was a reverse foreshadowing about Sin being Jecht, who was a man from an OtherWorld (in the words of Maechan
)
I really doubt that if the dream zanarkand was a physical place that the hymn of the fayth would be heard. It was hard enough to hear it just through Sin's armor.
(back to the city inside sin, it could just be an apparatition of pyreflies with tidus' thoughts, or possibly with yevon or jecht's. They do that sort of thing. )
I'm sure if Dream Zanarkand was a physical place Cid would've added a spot to the airship saying 'omgomgomg IT"S ZANARKAND LIKE IT WAS 1000 YEARS AGO! omgomgomg!'