Sorasoren

Can I use Johto/Kanto pokemon for the Hoenn Elite Four? From my understanding you cant go back to Johto/Kanto until you beat the Hoenn Elite Four, but does that mean I cant use the older pokemon either?

I get a different answer every time I ask this question so I decided to come to the forums and get a final answer. Is it true that if you level an Eevee on love island (kanto) that it will evolve into Sylveon?

Where can I find a dusk stone? Just caught a Murkrow today and I am super excited to evolve it, but I cant find anything on how to get a dusk stone.