First, those theories are wrong. We kind of know that Ava did change the future, because why else would they make such a big fuss about it, but let's ignore that for now and consider both possibilities. If she did change the future, then what she did is not in the book, therefore the Master of Masters didn't know about it, so he cannot call her a traitor. On the other hand, if she didn't change anything, then why would it be a big deal? Yes, she did disobey MoM's rules, but so did everyone else. Forming and breaking alliances, spying on each other, trying to take over and become a king, planning to summon Kingdom Hearts...
From that we get one possible answer: All of the foretellers are traitors. They all violated some of the rules, and I think at least two of them actually believe themselves to be the traitor mentioned in the book.
But there is more.
In the biblical stuff (Should we call it satire? It feels a bit too subtle for that word.) MoM represents God and Luxu represents Lucifer, the traitorous angel who started a rebellion and ended up becoming the Devil. Luxu broke an explicit promise (that he gave in Back Cover) by opening the box and reading the book. It seems that in future games the main conflict is going to be between these two. So, obviously Luxu is the traitor.
But there is even more.
These days many smart people think that God is actually the villain of that story. He claims to be absolutely good, but his actions are (at least by modern standards) those of an unquestionably evil being. Lucifer is just pointing out the Truth about this, which is generally the right thing to do. And the same logic applies to MoM and Luxu, think how much death and suffering MoM has caused. This is a good reason to say that the real traitor is actually the Master of Masters himself.
But then there is even better reason.
When MoM talks about his role in the previous keyblade war, he is doing it in a way that intentionally has a double meaning. He doesn't want to lie, but he doesn't want to tell the truth either. He says that "the monsters looked like us", because he was one of them, likely even THE one. He says that other keyblade wielders didn't count, because they were fighting against him. He was on the side of darkness in that conflict! Then he changed sides at some point, becoming a traitor in a very literal sense. Now he is a fanatic of light, deeply ashamed of what he once was, hiding the sigil under his black coat. Some further evidence of all this is in his symbol, where the wings of darkness and light describe his transformation and form the X-shape of the sigil, but it is hidden behind a cross, which represents what he is doing now.
That's three interpretations, all valid in some sense. Of course there may be even more...
So, "Now that you know who the traitor is, can you accept it?"