I'm pretty sure none of them (except Auron obviously) knew about the Guardian being the Fayth for the Final Summoning.

https://www.youtube.com/watch?v=dyCb6-aP3lo

Exhibit 1: Yuna is obviously confused and highly distressed that the Fayth is vacant.

http://www.youtube.com/watch?v=dyCb6-aP3lo&t=3m25s

Exhibit 2: Yuna's silence is because of obvious confusion which necessitates Yunalesca elaborating that she must choose the one to become the Final Fayth.

Exhibit 3: Everyone's gasp of horror at this pronouncement.

Also Tifa's Boobs, there is a simple reason for why Seymour's mom did that to him-
she's terrible.

. Just listen to the nonsense she spouts when you get Anima.

Seymour's Mother: And so I becae a fayth. But... Because I let him taste power, He began to thirst for more. He was not satisfied with my aeon. He wanted more. More power.

Or maybe you scarred him for life.