Arceus Mew Theory
So, I've been thinking about the contradiction present in Arceus or Mew being the first Pokemon. So, I think I've come up with a theory that satisfies both of the explanations of how Pokemon and their world came to be.
-Arceus is hatched and possibly creates the universe
-Arceus then creates one of two Pokemon groups;
Either Palkia, Dialga, and possibly Giratina, then Azelf, Uxie, and Mesprit. This is the case if Dialga, Palkia and the lake trio are the creators of space, time, and willpower, knowledge, and emotion.
If they merely control these things, then they more likely came after Mew, because Mew is believed to be ancestor of all Pokemon. So, Mew would be created and used as a basis for all Pokemon, including all Legendaries. So, many Mews are then made.
-The Pokemon world is then either created by Arceus, or is already existing and chosen by Arceus to inhabit the other Pokemon that will soon exist. If it's created, then Groudon, Kyogre and Rayquaza are also created if they are the reason for land, sea, and ozone.
-Mew is then used as a base for all Pokemon. Therefore, Arceus creates every Pokemon (possibly excluding Legendaries) using each Mew as the "clay" that is molded into every Pokemon, which is possible by it's ability to learn any move of any type. It's type is granted by Arceus changing type and inheriting it to each newly-made Pokemon. All Pokemon are then created.
Therefore, Mew is ancestor to all Pokemon, and Arceus created them all using Mew.