That's true about Mew, but I believe that it was also on one of the early posters for the movie. Other than the brief trailer that was shown at the end of the Arceus movie, I don't think that I've seen Lugia or Ho-Oh on the other poster with Celebi and the Legendary Beasts. Though, if they do show up, I agree that they'll be main support Pokemon or come in at the last minute like Kyogre in the Manaphy movie and Regigigas in the Giratina movie.
Since Celebi and the Legendary Beasts are on a movie poster, I'm pretty sure that they'll play some kind of role in the movie. I'm not so sure about Evil Ash showing up for some reason. Though Zoroark being able to create illusions does make it a possibility. If it does show up, I think that Evil Ash will appear in a way that isn't what most people expect it to be. As for events in the trailers that didn't happen in the movies, I still remeraber seeing Deoxys land in a desert with Ash being there for movie seven and Lucario somehow getting hurt, along with Pikachu falling off a cliff, for movie eight. I'm not really sure if Heatran was added into the main plot for movie 12 since it didn't play a significant role like Darkrai and Shaymin did in their movies.
Still, I agree that we don't have a lot of solid information about the movie and we probably won't until spring.