I thought about both of those, but Bahamut already covered the "mind control from the moon" in offering Nael much-desired power.
As for Kuja, I didn't think it'd be right for a man of any Eorzean race to have a "creator" unless he's an undead Frankenstein monster of sorts, which would likely not make it to Legatus status anyway. Perhaps instead of "creator" it'd be his parent, and whatever shortens his life might be a hereditary illness. Either that or they had him enhanced at the cost of halving his life span.
