Here's the big question though: What if Pokemon scientists are WRONG about Mew? The assumption is that because Mew possesses the DNA of all Pokemon within its own, it must be the ancestor of all Pokemon. But add in the other unique thing about Mew, its ability to learn all moves. I think the only reason Mew has the DNA of all Pokemon is so that it can use any move it wants to.
Arceus, even in-game, shows evidence of being a "god" because in HG/SS, it can create one of the Legendary Sinnoh Dragons. Not to mention it being the highest level wild Pokemon to date and having the highest stats of all Pokemon. I believe Arceus created Mew that way, not so it could become all of the Pokemon of the world, but just to make it unique.