The MoM is male, right? So he couldn't be Ava. But I do think it's really cool we have another female character with depth, and who is powerful and whose proactive actions have a huge impact on the plot. And this game gave us Skuld as well, so I'm extremely happy with it.
Especially because Ava's motive for doing so I don't think was very good. She wanted to preserve the keyblade and life be better off if the keyblade vanished forever.
But without keyblade wielders there is no way to defeat the heartless, right? So aren't they needed?
Also, I got interested as to where the name Anguis came from, as it wasn't a translation of snake in any language, so I looked it up. It turns out it's actually a name for the slow worm, a type of limbless lizard that resembles a snake. Probably doesn't mean anything, but it's still interesting.
I agree with all the points made about the writing. It's amazing (which I definitely wasn't expecting in the beginning) and I hope they can match the quality in KH 2.8 and KH 3.
Thoughts on the plot:
So it looks like Ava isn't the traitor (or at least an actual one, she might still be considered by the other foretellers to be one) like it seemed before we got translations. I think she's telling the truth about stuff. It seemed to be a big end-game reveal, so it would be rather detrimental to the plot to say "just kidding". Plus a lot of the clues seem to fit, and there is still lots of mystery, which is almost always the case when actual reveals are made. Answers in this series always lead to more questions.
It looks like the player still hasn't made it to the tower. Though now there doesn't seem to be an in-game reason to go there so we probably won't find out what's in it, at least in X. I bet it still holds some secrets though. Maybe some of them will be revealed in Back Cover, since that focuses on the behind-the-scenes stuff the foretellers are doing. (Also, maybe the illusion of the tower was created by Ava's tome, since it can create illusions of places. Or can it only create illusions of worlds? If it was the tome that was responsible, and it enabled Ava to change her appearance as well, then it has bad implications, ones that make the theories about heartless actually being illusions of other players seem even more plausible. But then I'm not sure that really fits in with the direction they seem to be taking things.)
If there is meant to be a trio that Skuld and Ephermera are a part of, I would say that the player is the trio's third member. Though I would prefer if that wasn't the case, as I would rather they didn't make every friendship into a trio. That's been done enough, in my opinion.
Lots of new things to spectulate over, which is exciting.
