Do you need to complete the Unova pokedex to get the Nature Reserve?

Question

Can you get it in the US games?

Answer 1

Yes you do. And Yes you can get it in the US version of the game.

>If you see all of the Pokémon within the Unova Dex, when you go to talk to Professor Juniper, she will notice that you have seen all the Pokémon within the Pokédex and will offer you a Present. This Present is the special Permit item which takes you to the Nature Preserve where loads of rare Pokémon, including a Shiny Haxorus, dwell.

Source

Answer 2

Yep. It is the only way. You only need to complete the Unova dex

Answer 3

Yes.You need to see all the Pokemon in the unova dex.Victini,keldeo,meloetta and genesect are exceptions since they are event Pokemon.You can get it in the US games.