What if the skull with the bullet hole doesn't have a bullet hole at all, but rather an exit point to a flying filling! It's Alvarez, and the bodies in the pit are not entirely from the purge, but rather the reason the purge happened at all. The pit is started by dharma hiding thier own dead, and possibly experimenting on them for some form of the life-extension project. The perverse nature of those experiments led to the purge, of which i'm beginning to feel more and more that Ben did not have as big a role as he would have John (and us) believe. Just a theory obviously but please let me know what you think!
Kelvin says Radzinsky put a shotgun in his mouth, which would have blown apart his skull rather than leave a neat little hole.
Shotgun shells beings fired up close "in the mouth" may result in a circular wound. Compared to it being fired from a range, which will make the shells spread.
Kelvin was lying
Kelvin was not lying, Cuse and Lindelof have confirmed that Radzinsky did die in the Swan from suicide. Due to the blood splatter on the ceiling of the Swan, it is likely that he pointed the shotgun upward inside his mouth whenever he pulled the trigger. If that is the case, there would not have been a gunshot wound in his forehead as his head would have been mutilated from the blast.
The Body with the gunshot wound to the head was Olivia Goodspeed, due to similar clothing and traces of blonde hair attached to the skull.
The body with the bullet hole in the skull is Sawyer. This would explain why the camera specifically focuses on the body, as if it was important.
The others left the grave open intentionally, because things that are buried on the Island have a way of not staying buried (Locke quote to Paulo).
The grave is open as the Others never dug it in the first place. They simply dumped the bodies in the pit, which was created by natural causes.
The grave is square and therefore unlikely to be a natural formation.
The grave is open, because Ben used dead bodies to put in the wreckage of the plane in the ocean.
The grave is open because after the initial purge numerous other victims were added to the pile of bodies, such as the person with an apparent bullet-wound to the head.
Ben never intended John to die in the grave. Ben knows that the island will heal his gunshot. Ben uses the grave to help John understand that no one else should come to the island.
The grave isn't what it originally looked like, since Ben or someone used the corpses for the wreckage mock-up.
The gun is clearly Locke's, and fell out of his hand when he was shot.