I always thought that the dream Zanarkand was high above Mt. Gagazet, right above the mass of fayth. Because in FFX-2 (SPOILER)the clouds all disappear and that new place opens up, and I figured that was the dream Zanarkand going away.

Anyway, if you go talk to the fayth after you get the airship, they will tell you some of these things. I don't think that Jecht was chosen to go, I think it was an accident. Tidus was chosen because of Jecht, but I don't think Jecht was chosen. I always thought that Sin went to the dream Zanarkand so that Yu Yevon could see it. That's what all of this was for, I'm sure Yu Yevon would like to see the fruits of his labor. I think Sin attacks the dream Zanarkand because of Jecht. Jecht has moments of control over Sin, and destroying the dream Zanarkand was one of those moments. I don't think Yu Yevons would ever have done that.