With regards to the demo, I think there are 3 main possibilities:
1. There are no more Mega Evolutions.
This is probably the safest, least assuming prediction. The demo code showed no extra mega stones or evolutions, nor did it really have any obvious place for them. It's pretty likely there's just no more.
2. The demo is based on an earlier build of the game, and more mega evolutions were added after this build
While this is also a good explanation, it seems to have some holes in it as far as I can tell. As far as I'm aware, Pokemon are one of the first things designed for a new game, not one of the last, so doesn't it seem a bit odd to have the whole game so fully polished, yet still be designing and implementing new ones?
3. The programmers intentionally obfuscated any unannounced Mega Evolutions in the game code.
This is the least likely explanation, in my opinion. Perhaps some people at GF found out people had learned how to decompile their game and unlock some hidden content (say they read any number of the stories about people having Bank mons before PokeBank, or AZ's Floette at all), and then decided to intentionally protect the game from those people. However, there's really no precedent at all for something like this, and it doesn't explain why they left in other stuff, like alternate-forme Hoopa.
In any case, we'll find out in a few days. No need to stress out over it.