If you complete the pokedex (incidentially, is it possible to get Celebi or Mew in RSE?) and then talk to the Game Freak girl in the inn in Lilycove about it being done, what happens?

the game freak girl..... gives you a certificate for completing your pokedex.
not much in my opinion.....
and.... u cannot get mew and/or celebi in RSE, using the game only.
u either gameshark or action replay(something we dont have in the phillipines. ) or go to one of those nintendo events..... u can also get celebi in a pre-order disk of pokemon colliseum.

You get a certificate for getting 200 in Hoenn or 386 in National...Do you get anything else for catching the rest of the pokemon?

I think in Emerald, if you get all the Hoenn Dex, and talk to every single person in the game, you can catch Jirachi, but I am not sure.

No, that's probably another rumor. For completing the hoenn dex, prof. birch gives you one of the jhoto starters.

i agree that what you get isnt all that great. a certificate. and you dont even get to "keep" it. if you want to look at it you hjave to go to the guy again and they will congratulate you and show you the certificate. it would be better if you could at least hang it on the wall in your room or secret base.