Ada is officially awesome. It's not often that a character can make Vincent both speechless and lose his cool. :D
Speaking of Vincent, he had a nice scene with Gil in his chapter. He can be kind of cute when he's not being overly evil. I'd like to see him be redeemed and for him to break away from the Baskervilles.
I suspect that Oz is the real Oz Vessalius and that no switch took place. Mainly because of Jack. I doubt that Jack would randomly appear in a nobody so Oz has to have a connection to the Vessalius family somewhere. When his father took baby!Oz away he probably did something to Oz to ensure that he'd be sucked into the Abyss when he turned fifteen. Either that or he did something to force Jack to make an appearance inside of Oz.