I like to think that Obra was pretty much a failure of an Etherious in the sense that instead of wanting to kill him, it loves him. Kinda cheesy, but I think it would be a sweet twist. It could also be intentional on Zeref's part, using Obra to change his outlook on life and kick-start the Curse.
Good point about Irene, and especially Brandish, I love the idea of young children being inducted into it, adds a sort of brainwashing element to it. It's also incredibly likely given how Ajeel, DiMaria and Brandish are all relatively young (I would add Larcade, but his age is unknown and he likely would have joined the 12 regardless). The two that I'm most curious about are Bradman and Wahl. Bradman's strange in the fact that you'd think he was created by Zeref, but he mentions the Underworld like it's an actual place and if he was an Etherious, wouldn't he be trying to kill Zeref, not follow him? I don't know, it puts doubts in my mind. As for Wahl, was he persuaded to leave his people, was he taken by force or did he seek Zeref out?