It seems unlikely that she is either dead or missing, as her diary makes several references to her forming friendships with other survivors, including a couple of the main characters. Her disappearence or death would have been noticed. It seems more likely that she simply lost her diary. The "bright fiery flash" was probably the dynamite that Jack, Locke, etc. used to blow the hatch open.
She went with Jack after the split because she is determined to get off the island.
When she thought she heard someone behind her she was really hearing the whispers and it was just too dark to distiguish between a person and the whispers she'd heard before.