Now that I think of it, it seems really possible that he might have let go of his hatred even before he was sent. When he was finally defeated, he seemed to accept that it was nigh time for him, and I could hear a tone of gratitude that it was
Yuna, and no one else, who got to send him in the end. I'd guess hard that he really did love her, in his own twisted way. He was never impolite to her, only to her guardians, Tidus to a greater extent. This is why I absolutely love Seymour/Yuna, and btw, I want to read that one-shot!
To the life-cycle point of view, summoning is highly unnatural. It may be a millennial art, but that doesn't make it less natural to the life and death order, in that it prevents the souls of the dead from going to the Farplane OR from becoming fiends: they remain trapped, as if they were still alive, retaining a consciousness of their own and something akin to a hive mind, only to manifest physically as aeons when a summoner beckons. Of course, because of all these implications, one could only be turned into fayth if he/she him/herself showed the
desire to become so, fully knowing his/her fate in eternal limbo. This may also be why Yuna states in the book that it's not the summoner who takes the initiative to bond, but the fayth.
I believe the "voice of the mountain" the Ronso heard was the Dream Zanarkand fayth. It is stated by Maechen that, when Bevelle troopers climbed Gagazet to reach Zanarkand, they found the city completely deserted and a multitude of the fayth singing the Hymn of the Fayth. The "voices" the Ronso hear from the mountain most likely stem from the fayth answering to them. But the game never mentioned anything like aeons that weren't manifested by fayth, just the effects of there already being fayth in the environment, such as the fayth from Macalania Temple (Shiva) causing the lake to remain frozen and giving life to the forest: when the fayth vanished, the lake melted, causing the temple to sink to the bottom of it, and the forest began to die slowly.
What was seen prior to the end of the Machina War was technology and summoning arts living together. There were MANY huge cities, not just Luca, Bevelle and Zanarkand: ruins of those lost cities can be seen spread throughout Spira, most likely destroyed by Sin. It isn't known if all those cities, allied to either side or not, had summoners, but Ifarnal is said to hail from Luca, so it's possible that the art of summoning wasn't exclusive to only Bevelle and Zanarkand. The whole thing that sparked the war was pure religious spite: while Bevelle had this polytheist religion hitherto unmentioned, the people of Zanarkand worshiped their ruler, Yu Yevon, and his family. And they fought with all they had, from machina to summoners, except the Bedohls managed to outrun Zanarkand in the arms race and Yevon found himself forced to do what he did, given his immense amount of control over the peoples' lives. This is why I don't believe the Bedohls ever served Zanarkand: have you ever seen a Yevonite Al Bhed, or a different sect of these separate from the Al Bhed we know? Although it MIGHT be possible that, if there really were any of them in Zanarkand, they most likely gave in to Yevon and became fayth for DZ.