OK, here goes. First, Cassie, it's ash, not powder. What it's the ashes of, I don't know. The reason the Others probably don't give answers to the Losties is because they were sworn to secrecy by Jacob. I mean, would you want your followers giving out the secrets of the universe, willy-nilly, to every Dick and Jane who strolled onto the island? Of course, Dogan did try to warn Jack about Sayid's pending evil. Jack just chose not to listen because he's a solipsistic, emo egomaniac who has been lied to far too often. That left Dogan helpless, because nobody who isn't a candidate can murder a candidate, Jacob and MIB included. I think the only exception to that rule is if that if a person doesn't know the other person is a candidate, they're allowed to kill them, as there have been plenty of killings.
Keamy was probably hired by Sun's dad. I remember him saying something to the effect of "if you attempt to run away from me with my daughter, I'll have you killed."