Dear Esther/WMG

The island is a place where grief-stricken spouses come to die
First off, the narrator and the protagonist are two separate people. The narrator's speech always seems to tell of things to come, not of what's happening (like the fall in the cave). What goes on at the island is people who have experienced the death of a significant other are drawn to the island, where they live out the memories of the first visitor. The voices heard are the previous visitors to the island, as shown by the whispers, like "Come back." not, "I should go back.", as in a mental opinion, but an outward command. The cars in the caves (at least 3) are the remnants of the car crashes of other husbands/wives. Also, when the narrator speaks of how he watches the planes in the sky, a figure is clearly visible looking up from a cliff above. When the people reach the end of their journey, they achieve inner peace and die, to be reunited with their loved ones.

The protagonist is dying throughout.
The entire game doesn't take too long to play - perhaps the same length of time it takes for the brain to completely stop functioning after a fatal injury; say, a car crash. As the game progresses, the island starts to make less and less sense, with the scrawled diagrams and quotations becoming more prevalent - perhaps the protagonist's brain is firing randomly, projecting snippets of memories into his consciousness, maybe including the island itself. There's also the fact that once the aerial is reached, he seems to become at peace, suggesting he's gone beyond caring (or reason). Of course, the game also ends with a flatlining EKG.
 * Confirmed, at least partially. When leaving Jacobson's farm, the Narrator says he has badly broken his leg, and the wound has become infected. "This will be my final ascent." The player character, if they're different people, has at least two hazardous falls, appearing to lose consciousness both times - and nearly drowning while having a flashback the second time, no less.

The protagonist is in Limbo
Just like Inception, you appear on a beach with no boat and no signs of how you got there, like a dream. The fragmented images during the walk around the island could represent the protagonist's repressed memories of Esther's death. At the end of the game, the protagonist hurls himself from the radio tower to wake up. The weird hissing/sucking sound you hear sometimes in the caves is the anaesthetic machine.

There are three narrators, and the player character is none of them.
The first is Esther's spouse, who begins his letters with Dear Esther, and generally seems to continue on the same points: Being acquainted with Donelly and Jacobson, having kidney stones which he believes have given birth to the island, and. The second is a man who speaks in strange flowery dialogue that doesn't make a lot of sense, he repeats some things spoken by the first narrator, such as "bottomless boats," but doesn't really talk as if he has ever been on the island, instead speaking to a "you" who may or may not be Esther. The third is implied to either be Donelly or Jacobson, and speaks to Esther as well, but refers to a "him" the first narrator. The third narrator's voice has a much deeper tone than the first two, making it the easiest to tell apart. However careful listening will have you notice the difference between the first narrator and the second as well. The third narrator speaks very rarely.

The player character seems to be Esther herself, as the narration seems more like it is addressed to the player than something the player says in reaction to what is happening. What that means is up to heavier debate as you'd have to talk about what exactly the island is.

The entire island is a simulation designed to help grief stricken individuals come to terms with their losses

 * Based mostly on the shadow people (the admins) and the odd robotic noises and radio transmissions you hear from time to time

The narrator died, and the player is his shadow

 * The other shadows you see are likewise representations of others. This helps explain why the story changes so much; having been detached from his life, the player can't recall everything correctly. There's only 1 time you're able to see the player, indirectly as a gull's shadow. There's also a moment in the caves where the narrator directly talks about his reflection, but the player can look into the pool and not see anything.