I think that the fayth that needed no summoner to control at Gagazet was for both Sin and Dream Zanarkand. I think that Jecht, Tidus, and every other resident of DZ was as real as Sin and the Aeons, and that whether they had been touched by Sin or not, they could exist and manipulate the real world in the same way as the Aeons. It's not confusing to me to think about when I put it this way, because it means they are real. The only question that springs to mind, though, is that after the fayth "dismiss" Tidus, if they had procreated, if at all possible, what wold happen to his offspring? Would they suddenly lose half of their chromosomes, be unaffected, disappear themselves, or what? How would a relationship with an Aeon really work?