Dunno if anyone has said something similar to this, cause I really can't be bothered reading through this whole thread. But anyway, this is what I think about Mew and Arceus.
In the beggening, there was nothing, until a giant vortex formed and unleashed chaos. From this chaos, Arceus was born. But, on the other side of the vortex, at the exact same time, Arceus' opposite, Mew was formed. Both wandered around in the universe, extremely bored, until they saw each other. Immeaditly from imense happiness, they both realeased their full powers. Arceus made all land, space, planets, regions and Mew made heaps of Pokemon. At first, it went well. But the vortex tore through the universe, making all pokemon unhappy and ruining the land. Arceus was also sad, because he was unable to make live pokemon. He/she told Mew, and they came to the aggrement that Mew would give Arceus the power to create pokemon that could control the universe (this done by Mew giving the power of making pokemon to Arceus) and that Arceus would give Mew his land powers (which enabled Mew to make pokemon that relate to the land). From this, Arceus made Dialga/Palkia/Giratina to control space and time, thus controlling the vortex. Now Mew, knowing that all earthly beings needed to be put through some strife in life to make them tough, asked Arceus to make a volcano, mountain and clouds, and from Arceus' land power Mew was able to create the legendary bird pokemon to control these mass destroyers. Arceus and Mew then used thier powers to cut up the vortex and put them in the volcanos, etc. Thus the world had been made and controlled.
My theory means that Mew and Arceus were made at the same time, and that Mew contains every dna because he created (almost) every pokemon. Arceus was able to make Dialga,etc. because Mew gave him the power to. Arceus is able to change types because he is the able to make land. Arceus and Mew are opposites, this is why they were born to other sides of the vortex and Arceus creates land and Mew creates pokemon.
Thanks! :)