Is it possible to find the hidden DexNav Pokemon (pokemon that don't usually appear on the DexNav, for example, Zorua on route 101) before defeating the elite four? Can you catch them as soon as you receive the DexNav?
You can't get them as soon as you receive the DexNav, but you can before the Elite Four. It's possible to find the hidden DexNav Pokémon when you finish the story with Primal Groudon/Kyogre. You just need to finish that bit of the story, and then go back to Professor Birch. He will then give you the National PokéDex.
̶e̶d̶i̶t̶e̶d̶ ̶b̶e̶c̶a̶u̶s̶e̶ ̶I̶ ̶r̶e̶a̶d̶ ̶t̶h̶e̶ ̶q̶u̶e̶s̶t̶i̶o̶n̶ ̶w̶r̶o̶n̶g̶