Well according to theories I've read, some of the alternate titles I've read for the movie is "Illusion Ruler Z" or "Illusen Champion Z", leading people to believe this is simply "Z" showing off his illusion powers by disguising himself as Ash. And with that said, it's also possible that "Z" can extend this power to disguise himself as Dawn, Brock, or anyone he wants. However it sounds like he can't change his voice... at least what the trailer shows (he most likely can, the trailer just kept his actual voice to "tell us" that he's not Ash (like it wasn't enough he was in shadow, had an evil glare, didn't have a Pikachu, and the actual Ash was standing a few feet away from him).

But to why he disguised himself as Ash is up in the air.

They can still be in the movie. Being Ho-Oh is the master of the Beast Trio, I'm sure if they are involved then so it Ho-Oh. Though whether Lugia and/or its Bird Trio are also involved is unknown.

*Points to his second post* We already talked about that.

isnt this the time where we come up with rediculous plot questions!?

Well if they were to wrap it up by June that means they'll have to do it within 28 episodes (if you count June, if you don't then that means their is 24 episodes left) without taking any breaks, and with the Team Galactic plot wrapped up all they need to do is:

1. Get Ash & co. to Sunyshore City.
2. Have Ash battle Volkner to get his Beacon Badge (2 episodes)
3. Have Dawn get her last ribbon. (2 episodes)
4. Have Dawn participate in the Grand Festival (3 episodes)
5. Have Ash participate in the Sinnoh League (7 episodes)

So with that marking 14 episodes down, that leaves 10-14 episodes left for them to get Ash and co. to Sunyshore, the Grand Festival, the Sinnoh League. Now unless they skip fillers, that isn't a lot of episodes to work with.

BUT WAIT! What about the Generation IV Battle Frontier? Well it's presumed that it is the next mini-arc that they'll be doing until Generation V is ready, however we don't know whether they'll be going to Johto or staying in Sinnoh for it. Thus we don't know where this episode takes place (and remember, we did see a Suicune in Sinnoh meaning the Beast Trio (and thus Celebi and Ho-Oh) aren't restricted to just their home region.