As such, Seymour's line comes off as one last desperate attempt to stop Yuna from killing him
I don't agree with this at all. Seymour says "So... you will be the one who sends me," and the tone of voice leaves no doubt that he says this with resignation. He's not pleading for sympathy; he knows he'd never receive any (and this is consistent with the fact that he didn't receive any when he was a child). His final comment is a defiant one; he believes that Sin can not be stopped, and defeat has not changed that belief for him.
Yoshiken: That isn't actually in FF9, or to the extent it is it's far too subtle. It's like bringing up Ultimania information; I want a complete story in game. (Meeple's reason is better, for all that it still doesn't work.)