According to Canon, Arceus predates mew. You refusing to believe that is you saying your own support to your theory means nothing because you are discrediting your own source. Both the games and Anime both specifically state that Arceus created everything.
And None of the Professors discovered any of the Legendary Pokemon. They collected data from old stories and myths. No one documented, except Dr. Fuji, discovered Mew, though they found a fossil of Mew that had its DNA that they used to create Mewtwo. The legends and stories of legends existed before the Professors produced the Pokedexes. Majority of the Pokedex information for almost all the Legends are from the stories and legends.