Complete Chapter 6.Story-related; cannot be missed.
Here is a walkthrough of this chapter, including all miscellaneous achievements, notes, logs, and murals.
Exit the boat and head forward along the path (Log 1) and approach the altar in front of you. Interact with the syringe on the ground (Log 2), then head through the altar to the base of the stairs where you'll find a gun (Log 3) and a pool of blood (Log 4) to interact with. Head around the right side of the stairs to the wall to find a mural (Mural 1). As you walk away from the mural, go off to the left to see a little camp. On the one crate is a case you can open and a picture to interact with, which you also need to turn around and read the back of (Log 5). Head back past the altar to the other side to find another camp (Log 6). On the crate to the left of the tent is another letter to read/listen to, and then go to the hanging cloth on the right side of the tent and read the rightmost page hanging on it (Log 7). Go back to the camp on the other side of the altar and go up the steps. Interact with the statue to put your hand inside (Log 8) (Note 1). Now return to the altar and interact with the top to open the gate (Log 9) and unlock Deep Cut (25G). Head inside the mountain door.
First head into the center, into the little puzzle pit, and interact with the gold stone in the center (Note 2). From the stairs where you entered this room, go to the right and interact with both doors (Notes 3 and 4) and the mural up the steps in the corner (Mural 2). Likewise, go interact with the two doors on the other side of the room (Notes 5 and 6). Return to the steps and go up on the two platforms at the base of the steps and press each button so two pairs of disks roll out to the pit in the center. With your back to the stairs, interact with the disk on the left closest to you, and press the only lit button. Then interact with the disk in the back on the right, and agian press the only lit button. This will open a door and you'll look towards it. Go through that door. As you go up the first steps, look to the right to the obvious mural on the wall (Mural 3). Head to to the top of the statues and use the statue to get a new symbol (Note 7). Return to the four disks.
Remove the two symbols you used on the floor puzzle so there is no light there anymore. With your back to the steps again, the back left disk, press the first symbol we used; the front left disk, press the new symbol; the back right disk, press the new symbol. This will open a new door. Head inside and make a sharp right up the steps to find another mural (Mural 4). Now head back down and go to the podium facing the water, with three black diamond buttons and one gold button. This part is tough to describe in words, but I'll do my best and use screenshots to help. First, press the middle black button to rotate the statue in the middle until it's facing towards you and off to the right. Then press the right black button until the statue off to the right is facing the middle statue, then press the gold button. It should look like in the screenshot below.
Go across the new platforms and up the steps to the second podium. Press the left black button once to rotate the far left ice statue once, the middle button to rotate the middle ice statue once, and press the right button so the right ice statue is pointed left, then press the gold button. Go back to the first podium. Press the middle black button once so the middle statue points toward you and off to the left. Press the left black button so the black statue way in the back on the left is pointed towards you and off to the left. Press the gold button. The platforms should form and look like this screenshot.
You should be able to walk across the platform to get to an alcove in the back left of this room, where there is a photo on the floor right in the middle of the platfom. Interact with it to unlock Candid Photo (20G). Now return to the first podium and press the left black button three times so the back left statue is facing you and off to the right, and press the gold button. Now you should be able to cross the platforms to get to the back statue and put your hand in for another symbol (Note 8). Return to the four disks and remove all symbols of light from the pit.
With your back to the steps, on the front left disk press the bottom right button, and on the front right disk press the same bottom right button to open a new door. Go in and interact with the obvious mural (Mural 5). Go up the steps and interact with the statue to get a new symbol (Note 9). Head back to the four disks and remove all of the light. Stand with your back to the steps. On the front left disk, press the upper left button; on the front right disk press the upper right button; and on the back right disk press the upper left button. This will open a new door. Head in and make a sharp left to see the last mural on the wall (Mural 6) unlocking Written in Stone (50G). Use the portal to go underwater, swim to the other side, take a portal up, go up to the statue to get the last symbol on your hand (Note 10), and then return to the four disks and remove all of the light from them.
With your back to the stairs, on the front left disk press the leftmost button; on the front right disk press the top middle button. This will cause you to look up at a scene in the sky (Log 9), unlocking Otherwordly Portal (25G). Remove the lights and with your back to the steps, on the back left disk, press the leftmost button; on the front left disk, press the rightmost button; on the front right disk press the upper right button; on the back right disk press the top middle button (Note 10). If you're gotten all notes, I'll make a note of that... (50G) should unlock right now. Head through the open door to the end, and look at the writing on the floor (Log 10) then look up and interact with the slimy mirror (Log 11). Walk around the corner and follow the path into the temple (Log 12). If you've been following along and gotten all story logs, The Amazing Adventures of Norah Everhart (50G) should unlock as you enter the temple. There are two endings for the game, and if you want to see them both, make a manual save now, especially because you must choose one specific ending in order to get an achievement.
Follow the chapter to completion until you get to the point where you need to choose a door to go through, deciding your ending. To have access to a final achievement, you'll need to choose to accept your fate and leave Harry behind. When you go through the door, you'll play the music box, and then have a stone walkway to follow. Just before you reach the stone rings in the sky, stop. The screenshot below shows approximately where.
Turn left and walk out on an invisible path towards a rock floating in the sky. Just push around the edge of the walkway until you find the invisible path. The screenshot below shows me facing the rock with my cursor pointed at it.
Walk up to the rock and interact with the key on it to unlock Meta-reference (25G). Now just follow the paths to completion. You'll need to wait for the credits to end and see some more dialogue, at which point The only thing I have now (100G) will unlock, and you're now done with the game!