Okay, so about the burnination of the plane. First time around, I was really upset that the plane was there and looked the same. After seeing the inside, there is some smoke damage and I'd understand if the bodies were burned to a crisp and the bones carried away by animals (which Obi Lockenobi suggests). The Virgin Mary is still intact, however, which irks me but whatever.
ANYHOW. I noticed a disconnect between the Smogmonster's appearances, and the Smogmonster's killings. Every time either Locke or Eko stood up to the Smogmonster, they had sticks in their hands. Locke had his walking stick, Eko had his Jesus stick, and in this last episode the first time he saw the Smogmonster by the river Eko had a stick he had wrenched up from the ground in his hand. When he died, and everyone else who has been killed by it, had nothing in their hands. I think if you are holding part of the island, the Smogmonster cannot hurt you. Hence, it is the servant of the island.