Possibly, the original battle between Groudon and Kyogre resulted in the forming of what would become the Hoenn region (since half is land and half is ocean). They didn't "create" the region specifically to create it, but it happened as a result of that battle. At least, that seems to make since to me.
This one, I don't know. There isn't anything I can remember from DPPt to suggest that Dialga and Palkia created the Sinnoh regions. Still, most of the people of Sinnoh seem to believe it. Sinnoh seemed to have more tributes for Dialga and Palkia around the region (the statue, the cave triangle, text books) than any other region for their Legends.
Remember, Cyrus is the guy who wanted to create a world for himself. He was half crazy. He would be one of the last people I would think knew what he was talking about.
And the fact that the Distortion World remained unaffected by Giratina's defeat/capture could be evidence that it was not responsible for its creation, which points back to Arceus. Perhaps if that dimension was completely controlled by Giratina, it would have been destroyed.
Arceus is the only Pokemon we know that can do this. Plus, the Sinjoh Ruins were specifically created for Arceus. It seems very odd that any Legend would be able to do this. Not only that, but Arceus doesn't produce another Arceus through this event, only 1 of the 3 Dragons its legend states it created.
My own personal theory is that the Unown are Arceus's power manifested (Arceus can be any type, and so can Hidden Power, which is the only move Unown can use). The Unown are mysterious and have never been a part of anything important in the games until the Sinjoh Ruins event.
Actually, I like discussing this topic because it's fun for me. I like seeing different theories put against each other and seeing which one has the most credit to support it. Game Freak intentionally leaves holes for the fans to fill with their own theories, so it's interesting to see which theories are the most thought out.
This is only stated in the Pokedex, and it is only stated as a theory by some Pokemon scientists. It is not a fact that Mew has the DNA of all Pokemon, nor is it a fact that Mew is the ancestor. And there isn't anything in-game that suggests it.
To be fair, this is not outright proven either. It is highly implied, but isn't stated as a fact.
This is the error I see most people make, they want to say that Mew or Arceus is God or a god. They aren't God or any kind of god, they are both just really powerful Pokemon. People attach their own definite of the word "god" to these two Pokemon. The way I see it, the Pokemon universe has no god, only an extremely powerful Pokemon that the legends of the Sinnoh region claim had the power to create the world and possibly the universe.
Here's the thing, they both can't be the first Pokemon. 1 of them had to have existed first, but you're assuming BOTH of their theories are true, which contradict each other in that case.
One Pokedex entry says that the "DNA of all Pokemon" theory comes from Mew's ability to use any move. They know it can use any move, and that is their theory on why. I think that just makes up for Mew not having its own signature move, ability, or special trait, like Celebi's time travel and Jirachi's wish granting. Knowing all moves I think serves as its special trait.
Like I said, I don't consider Arceus a god. Even the most powerful Pokemon in existence is still a Pokemon in the end. Even if GF were to create a new Pokemon with 1000 BST, it would still be a Pokemon like the others. Arceus is limited for battling purposes in the games. (It can't be unbeatable. That would defeat the purpose) So they program it with limitations, such as its Multitype can only let it change through the plates. And again, I think the Unown could serve to house that creation power that it uses in the Sinjoh Ruins event. (Though, that's just my theory. It doesn't need the creation power anymore because the universe already exists!)
We can't just say "If it created the universe, it shouldn't be able to be captured!" Why not? It's still a Pokemon. ALL Pokemon can be captured, regardless of their power.
Mew knows what the other Pokemon will do? I've never heard that before.
In the movies, Arceus can change type at will, meaning all it would have to do is become a Dark-type and Mew and Mewtwo's psychic attacks would be useless against it.
Also, Shaymin was no match for Giratina in that movie, yet Arceus defeated Dialga, Palkia AND Giratina. 3 Legendary Dragons couldn't stop Arceus.
That's a very odd theory. If Mew can't create the universe, how could it possibly have the power to create a creature that can?
In Dragon Ball Z, the Eternal Dragon couldn't grant a wish that was beyond the power of the creator of the Dragon Balls. I think the same thing would apply here. (or in general)
Again, you're trying to force the idea that both theories are valid at the same time, which is why you are left with the "What created Mew then?" question. Mew being the first Pokemon and Arceus creating the universe can't both be valid. This is where I like to look at in-game references for the both of them. If you ignore the Pokedex entry for Mew, you will not find anything suggesting it is the ancestor at all, as the Pokedex is the only thing that states that theory. Even the Pokemon Mansion documents about Mew's discovery and Mewtwo's creation don't mention it being the ancestor.
I'm done arguing I seen it coming by someone who puts down any theory anybody has to say if they don't agree with it. This was exactly what I was talking about. You completely slapped my theory in my face.