Help me please!

I was wondering if you could go back to visit islands 1, 2, and 3 after you already visited Bill. I did as soon as I beat he 8th gym, and I want to go back to one island to catch Moltres. Do I have to beat the Elite 4 first and then I can visit it? And I was also wondering if the Elite 4 are at the end of the victory road tunnel, because I've been wandering around everywhere and I'm stuck. Are there any tricks? Thank you!