Of course, all of you are welcomed! The more the merrier! The IC/OOC will hopefully be up in a day or two, too, after we've seen some character sheets popping up.

As for you Antarctic Termite, but I'll have to answer the majority of your questions with a fairly simple explanation. 
Whilst I appreciate the fact you've thought of some pretty extensive methods of justifying how I've presented the world in contradiction to the preset canon, I'm afraid it's really just a case of this [i]not[/i] being canon.
There was never a war in Orre between the allied forces of Kanto/Johto and Sinnoh, but because this doesn't strictly take place in the established Pokémon continuity you're familiar with, we can say that there was. 
Giovanni also wasn't alive in the '20s, unless the man's got a fondness of shouting "There can be only one!", and being struck by lightning: I believe the Pokémon television and game series are presumed to have taken place in the 90s and early 21st century, meaning that this game probably predates Giovanni by, at the very least, fifty years or so. 

The reason I've set the roleplay based around the four regions I have is because the likes of Johto, Kanto, Hoenn and Sinnoh are just rural enough for it to be believable that they went through a period such as this. 
After the Sinnoh-based games, the cities simply became too technologically advanced to justify them existing in the same world as this shady, Noir story: Comparing Unova to Kanto, it's impossible to believe that, even back in the 20s, they would've have been far more advanced, totally ruining the sketchy theme Goldenrod has going for it. 
Imagine watching "The Maltese Falcon", played out exactly the same, but in the shadow of 21st century Tokyo. It ruins the ambiance, no? 

"Leveling up" has yet to be truly discovered in this world: Just as we didn't always judge intelligence by an intelligence quotient, these people didn't always judge a pokémon by "level" or "experience", because at the time we had no way of measuring it. 
When a pokémon evolved due to leveling, it was just presumed that this pokémon had hit it's adolescent period, so to speak. 

Type-based immunities are still present (although it'd be very nice if you perhaps roleplayed your trainer being surprised by this, as, of course, this predates the all-knowing pokédex and the modern-day pokémon professor. Your trainer will only know these things from experience).

Because you can only carry four pokémon at a time, triple battles seem a little far fetched, don't they? Double battles can still occur, but even that's rare: That is, unless you're partnered up with another trainer, in which case they're a possibility. This would make for some lengthy collabs, however...
If the move was buffed after Gen 4, then I'm afraid it'll remain unbuffed for this roleplay. 

In summary: It's not canon, it's just meant to be a fun new look at the series. Whilst I appreciate the enthusiasm and interest, please don't bog yourself down in semantics, I just want you all to enjoy yourselves, you know?

There's no rule saying two trainers can't have the same Pokémon, so go right ahead! Just be sure to make 'em unique!