While I do, like you, think that Z-Moves do have something to do with Zygarde, I don't agree with part about Pokemon Z. As far as I know, Pokemon Z was just something that people thought that we might get after X/Y and it was never actually confirmed and later cancelled. For your theory to have some chance of being correct, that means that it was confirmed that Pokemon Z was going to be a thing, but it was cancelled, right? Unless, of course, it has been confirmed that Pokemon Z was going to be a thing but it got cancelled and I'm just behind on things.