I know this isn't the only accepted conjecture, but many of my friends say that they all were dead from the start. This simply isn't true. All of the characters eventually died, as they were seen in the church, but weren't dead from the plane crash. I don't believe that the island was Hell, or even a testing ground for the so-called "Sideways Universe." The island was just that, an island, even though it's a strange one at that.
Christian said that everyone dies sometime. Meaning that all the characters didn't have to die in the crash, they just had to die. And once they were in heaven, they were happy, as heaven is perfect. Also, how could the "dead" Kate, Sayid, Jack, Hurley, Sun, and Aaron(who wasn't even born before he "died") communicate with the "living."
The pockets of energy are simply areas with large amounts of electromagnetic energy, as we know from the series. The energy caused space-time to stretch, like gravity causes it to stretch. The bearing of 305 degrees was a way to enter the magnetic field without going through stretched space time. And, the wheel that the Man in Black helped create had metal on the spokes. The metal can be used to manipulate the electromagnetism by conducting it and altering the field, creating a new "safe" path to leave the island by, resulting in a fast-track off, to somewhere else(like Tunisia). Also, Faraday had to find a new bearing depending on what year it was. This makes sense considering that space time is always changing, causing a change in the field. The stone plug that was removed by Desmond must have had some sort of metal, keeping the field going and the Man in Black from being able to leave.
As to why the Man in Black and Jacob are immortal, that's beyond my imagination.