Why would the main villain blab his plan? 'Oh hey Yuna when we get there, um one of is gonna have to turn into an Aeon. Um, I totally do that.' That wouldn't seem a little bit weird? Wouldn't it be weird if Seymour started to go on about unsent, BEFORE HE KNEW THAT YUNA KNEW HE WAS EVIL? He was trying to manipulate his way into power, you don't do that by going on long rants. If you look back all of his actions would lead to Yuna getting a Final Aeon, its not tough to imagine that he had this plan for a while. In fact there's a line where he directly states, "No the pilgrimage would continue."