It was said that some dna of every Pokemon was found in mew, meaning that the others had to come first.Also, if you were to look at the pokedex entry, it says that Arceus emerged from its egg into nothingness.Not any other pokemon was there as it created the pokemon world.What you have presented is an interesting case, however, as it could be possible that the strains and mutations of Mew's dna could have created Arecus so that he could create the rest.Either way, I simply stick to the one that is most likely true.
