I believe Maechen explained about the summoners volunteering to become Fayth after the war. That is, the Fayth in the temples around Spira; the Fayth Cluster on Mount Gagazet were probably all regular folk.

Seymour's mother is an apparent anomaly to this.
Have you seen the good ending of FFX-2?
Not yet. I've seen that 'sad ending', I think - the one where (SPOILER)Yuna says goodbye to Tidus in the Farplane Glen, but I'm still working on that '100% completion'.