So I was rewatching "316" and I had an idea...
Could the flight number be a foreshadowing of the story of whats to come?
John 3:16 "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life."
Throughout entire show the main and theme has always been belief. Locke is a man of faith and Jack is a man of science. In "316" Jack was once again confronted with the issue of faith, this time by Mrs. Hawking, Ben, and Locke.
Perhaps believing in Locke brings about eternal life. That would explain Richards immortality.
For God (the island) so loved the world (people on the island), that he gave his only begotten Son (Locke), that whosoever believeth in him should not perish, but have everlasting life.
The island is like God and the purpose of the O6 getting off the island was to bring dead Locke back to the island. My theory is that Jacob is Locke's spirit. When Locke's body gets back to the island it will be resurrected as Jacob. It makes sense because when they returned to the island it was in the past, therefore Jacob/Locke would have existed before the Losties ever made their initial arrival.
It also makes sense because when Christian Shepard's body came to the island it also became a spirit that sort of represents/works for the island.
So to summarize...
-The island resurrects dead bodies that come from the outside as spirits.
-The island used the O6 as a means to bring back Locke's dead body so that he could be resurrected as Jacob.
-Whoever believes in Locke/Jacob/the island has eternal life, hence Richard.
Just a theory. I hope that makes sense.