That much I got alright, what I didn't get was why he was so keen for Yuna to beckon Tidus, even going to the extreme of killing him so it
would be possible for her to do it. It just... doesn't fit. :/ It's pretty much like the reason for summoning the island at all.
I think Johit is a really sad man. He said that he wished he had died much earlier (perhaps during that battle?), that he regrets being so vain in his youth (he
really looks like Bickson in my mind, that jerk from the Luca Goers) and his unrequited love for Kush. Maybe he had no other purpose than to summon, after his expulsion from Besaid when the Church built the temple there... pretty much like Yu Yevon, the very man he had fought against during the war. At the same time as he felt jealous of Kush and Valm/Bria, he'd erase the latter's memories about the former maybe in an effort to protect his existence, since that was what Kush would have wanted: to set Bria free from her and the war. He had a whole future ahead of him and only death awaited her. She chose to get out of the picture so he would leave, but that didn't work out and the "mechanical" Bedohl killed him.
Speaking of mechanical Bedohls, are they really a thing? Because the ones we saw in the story were actually
true, human, only that Alb might have done some magic or brainwashing in them so they'd act "robotic". In my mind, they look like
this, which is actually an Al Bhed kid. It's said that Alb had never managed to create artificial Bedohls. This should mean that the "mechanical" Bedohl we see is either a summoned one or an unsent that has been brainwashed.
As for the Aeon Cores, Ifahnal brought up a topic that's worth attention. The limitation imposed by Yevon on the post-war summoners have severely diminished their potential power. The knowledge of creating fayth has been concentrated into Yunalesca's hand and any other methods may have been forbidden by the teachings. If, say, Ifahnal had passed down his knowledge, he'd surely be executed and sent, and that was the last thing he wanted. The utility of summoners had been reduced to "defeating" Sin (that's what people wanted to believe in) and perpetuating the cycle whilst injecting false hope in people. Forcing summoners to use whatever fayth available to them and deny them the knowledge of creating their own aeons severely diminished their power. The Aeon Core was akin to the Final Aeon Yunalesca created - it was an impressive creature born from a fayth who shared a close bond with its summoner. Imagine if anyone could harness such power - the Church would be powerless. It could be said that the Yevon era was, indeed, a Dark Age.