It’s not so much that Gandalf is more powerful as the White, as that he’s allowed to use more of his power, and more directly. And his final confrontation with Saruman was more a matter of authority than power. It was never his role to directly confront the evil things of the world, but to inspire others to do so. But the situation’s starting to become desparate at that point, so he’s given leave to act more directly.
The Dwarves are the adopted children of Illuvatar. When Aulë made them, they were nothing more than clever automata or robots, but when he repented his pride, Illuvatar instilled them with the Secret Fire (approximately analogous to the Holy Spirit), thereby making them “real”, and now that they exist, He loves them as all His children.
There are definitely evil elves, or at least elves who do evil. As I recall, the fellow who forged Gurthang (Turin’s sword) was guilty of kidnapping, rape, incest, murder, and suicide, and there’s no indication that he ever repented. And every High-Elf in Middle Earth is a kinslayer or decendant of a kinslayer, save for Galadriel (and she could well be considered an accomplice), but most of them have repented and been forgiven. They do indeed go to the Halls along with everyone else, though they might have to serve some sort of pennance, and it might be longer before they’re reborn (it’s implied that Feanor won’t be reborn until the End).
Outside of the bounds of Middle Earth, by the way, you might also read “Leaf by Niggle”, a short story by Tolkien, for some of his views of the afterlife. Wait, no, never mind that, it’s obviously not about the afterlife, since that would be an allegory, and Tolkien cordially disliked allegory in all its forms. But it’s a story which just happens to coincidentally bear a striking resemblence to a story about the afterlife.
