You won't need this - if you've watched the first half then, yeah, what you've guessed is probably right.
So rather than cut to the chase, Jenna recounts the story from the beginning over a few days in the hospital. The good guys die one by one. Keith is found hanged upside down, one girl is locked in the hot tub and drowns, the blond brother steps on a trap and then disappears, the other brother is shot with a nail gun, but is still ok.
At the docks, trying to find the blond brother, they realise that Megan is innocent so should leave and go her own way. They ask the older brother about what happened last year and piece together the name-clues that were left to spell "Regina". The brother tells the story of a girl they got drunk and slept with, but then as she was leaving she tripped and hit her head. She died from the wound, so the two brothers buried her in the woods.
The three find the spot where the girl was buried, but instead find the body of the younger brother.
The others return to the cabin, where the older brother opens a cupboard to be bitten in the chest by a snake and soon dies.
Mark & Jenna venture upstairs to find the decomposing body of Regina - and then Keith (still alive) appears and holds Jenna hostage. Mark shoves Keith over the top bannister, and breaks his neck in the fall. Megan shows up, and the two girls run down to the docks - where the boat has reappeared. They get on board.
The detective at this point raises a few questions. Jenna says that Megan then picks up a knife, and Jenna has to kill her in self defence. The detective buys it. The psychologist isn't so sure.
Jenna goes back to the hospital bed to wait for her parents. She drugs the psychologist and escapes wearing her suit.
Jenna's parents arrive - and point out that their daughter, Jenna, is in fact who we thought was Megan. Megan was Regina's little sister.
Jenna/Megan a.k.a "Keyser Soze" ditches her ride and hitches a ride to escape to freedom. It makes sense then that she can retell the story without even being there as it was all made up anyway.