[q=Skogs]I don't think that Fayth have to be summoners. Reasons being: Bahamut's fayth seems rather young, Seymour's mother isn't a summoner as far as we know and guardians become Fayth for the Final Summoning.
[/q]Sure, but all the 'mainstream' Fayth are, or rather were, summoners - except for the Fayth Cluster on Mt Gagazet. Basically, all the Fayth that summon conventional Aeons were once summoners, the Fayth say as much in the game. Bahamut was simply gifted, like Lenne, Yojimbo and the rest. Age, in FFs, is little barrier to power or greatness.
Regarding the end of FFX, the party have to kill off the aeons so that Yu Yevon cannot use them to create a new Sin, as he would with any Final Aeon. One Sin is defeated, the Fayth can stop dreaming as the spiral of destruction has ended and Spira has new hope. Thus, their dream ends - taking the Aeons, the dream Zanarkand and Tidus with them.
Yep. Every time Yevon possessed an Aeon, he became vulnerable to attack, once all the Aeons were defeated, Yevon had to confront the party directly - and lost. Yevon, though, was also the driving force behind the Fayth's dream; when Yevon perished, so did the power of the Fayth. They didn't have too much of a choice, they were always bound to the power of Yu Yevon, the unsurpassed summoner. The Fayth of Bahamut explains about this, when you re-visit Bevelle temple.