You know as time goes on I'm more and more pleased with how Lost ended. I think the creators gave adequate clues to come up with our own conclusions about most of the mysteries, and let's face it direct answers to something like "what is the island" isn't nearly as interesting as coming to our conclusions. To me the island was the gateway to the spiritual realm and has been around since the beginning. Throughout the years various protectors were chosen, with perhaps the first being put in place by a God like being. Really I could go on but my point is that we didn't really need an answer to it.
The same applies to the Egyptian influence on the island. Now whether they came before the MIB or not is up in the air, since the evil that made up the smoke monster from the corpse of the MIB could have came into existence prior (explaining the skeletons in the cave with the light source). Basically the struggle we seen between Jacob and the MIB may not have been the first, nor the last to occur on the island. Or the Egyptian references to the smoke monster may have been created after the events between Jacob and his brother. Who knows, but the speculation is much more fun then getting an answer that may or may not be satisfactory.
The clues are there, that to me is all that matters, it reminds me of Stephen King's Colorado Kid, a detective mystery novel, it has not "answer" to the death but there are enough clues to make our own conclusion. I have little doubt the creators of Lost have their own answers to the questions raised. I mean if they wrote into the show that the cave and the mystical cork containing the essence of the island was created by an original civilization living there, like oh, the city of Atlantis, would that have been better? Don't think so.
Now about Walt, his task may have been completed when he got Locke out of the pit after being shot by Ben in season 3, because that set into motion the events leading up to the end. Perhaps he had a psychic link to the island, consciously or subconsciously and accomplished what he needed to.
Anyways It is neat to think of the events of Lost being from the perspective of those on the island like Spak brought up. It opened up a unique way of telling the story when we knew the people were flawed and were not always correct in what they said. Ben being the perfect example of saying things that were not necessarily true.