For a majority of these points, I'll use what I like to call the "flavor of the year". There were some fans who were behind Victini, thinking it would make it in due to all the promotions it was getting. Heck, I knew one guy who was *certain* Victini would be in, especially over Mewtwo, due to Mewtwo's "irrelevancy". But after Victini's movie came and went, its popularity dropped like a rock and Game Freak moved on to the next flavor of the year.
I don't see Diancie holding up for long. It's got a nice design, and a cool Mega. But these are all just temporary promotional pushes from Game Freak. They've been doing this since Celebi. Sometimes it sticks (Deoxys, Lucario, Darkrai, Zoroark, Meloetta, Genesect garnered pretty noticeable fanbases). But unless they give her another Mega, keep showing her in future games as a more commonplace Pokemon instead of an event, or have her frequent the anime, there's not much hope for her. She's relevant... for as long as Game Freak decides to keep her that way.
While Mewtwo's movie was last year, he is still pretty frequently showing up in their marketing material, like appearing in that recently opened Pokemon Center store as various items, or playing some roles in the games, spin-offs and main games alike. A very impressive feat, especially for a rare legendary Pokemon.
tl;dr Not worried about Diancie. The best she could show up as is a Master Ball summon. But not even Darkrai, Shaymin, and Arceus had that honor in SSBB (despite Darkrai's movie appearing a solid year before SSBB's release, and despite most fans knowing about them due to hackers). Her movie coming out so late hurts her more than you think.