First, Serebii fixed Diancie's Pokédex page: it can indeed learn Trick Room by TM. Still no confirmation on whether Trick Room actually replaces Power Gem in the level-up list, as well as whether or not it can learn Torment (not that anyone cares about the latter). But if Serebii actually changed something manually, it's not looking good.
But perhaps more interestingly is him spilling the beans on AZ's Floette, which was found in the game code a while back. Far from being filler data, or the second coming of the Notched-Ear Pichu, this thing seems to have been made as a more offensive counterpart to Florges, with less special defense, but more special attack and speed. Its stats add up to only one less point than Florges, which is to say, 180 more points than regular Floette. It's also the exclusive holder of the move called Light of Ruin, which, based on the description, seems like a less powerful but more accurate Head Smash, complete with 50% recoil. Not sure I like. Moonblast is such a sweet move as it is, I bet it'll probably remain the Fairy move of choice, especially since Stone Edge is often the move of choice versus Rock Head-less Head Smash.
Here's the problem, though: Floette/Florges' offensive movepool is trash. Moonblast, Energy Ball, Psychic. Not gonna scare a lot of people with that, and the Eternal Flower Floette (as it's officially known) has some speed issues as well. I guess it can try a Calm Mind set, maybe with Wish thrown in, but this family was clearly meant as a cleric... in UU, that is, because it's Sylveon's bitch through and through. Point is, what's the point of having more offensive stats if you can't attack properly?
Like the Notched-Ear Pichu, it's unable to evolve or breed, but one slight question remains: what about the Eviolite? After all, Floette as a species is still capable of evolving, even if this particular specimen can't. It all depends on how the game is programmed. If it CAN use the Eviolite, it might be able to go places (and in fact probably will). Otherwise, it'll be nothing more than a faster, but more frail and arguably weaker (thanks to Pixilate) Sylveon.
Finally, Serebii says it can only be obtained through an event, so expect to see AZ in a movie at some point this generation.