Sorry to say it dude but bout all the stuff you said is explained in the game and It all comes to diffrent answers. Like 1000 years ago bevvelle (the one next to the calm lands) had lots of machina almost as powerful as vegnagun(who by the way is so week in X-2 because hes turned OFF when you fight him) the destroyed Zanarkand and Yu summoned the DZ and used sin as a protection so he could keep summoning it and as a sort of revenge agains bevelle.