Also as an aside, I did report via twitter about a glitch causing the game to go in a state where all enemies despawned and checkpoints and cutscenes stopped working and, in case you were having problems working out what caused it from what I said since it wasn't the most clear, further routing to try and find the quickest route for speedrunning seems to have found the reason. At the start bit I had the video for doing it too quickly seems to not activate the checkpoint for getting the necklace and because you haven't got that you can't get the next checkpoint. Basically, skipping checkpoints does it. I tried doing things later in the run that would involve skipping checkpoints and the same thing occurred.
Hopefully that plus the zooming glitch are both useful to you