One Question That Needs Clearing Up...

[Christmas]

But if Sin is just an Armour, it doesnt need Fayth to exist. So why couldnt Yu Yevon become Sin, obliterate Zanarkand and once the survivers become the fayth couldnt Yevon begin summoning the Dream Zanarkand from within Sin

Why dun we look at it this way. If this can be done, then there isn't any point of creating Dream Zanarkand cause Yu Yevon can just create Sin and destroy the Bevelle guys so the Real Zanarkand will be safe...

We should remember that Real Zanarkand is under threat from Bevelle that is why Yu Yevon resort to this.

Also something to take note of:

Fayth: Yes. Even if you defeat Sin with the Final Summoning, Yu Yevon will live. Yu Yevon will join with the Final Aeon. He will transform it into a new Sin.

Now can the Fayth freely travel between Dream Zanarkand and Spira, without Sin's help? Bahamut's Fayth seems to be able to appear there at will, albeit in a noncoporeal form.

It appear so seeing how the Fayth appeared at the begining of the game.