Found the secret of the tower almost by accident, and this eventually led me to find the room with pictures of the three secret locations. Went to one of them, which eventually showed me a guy walking through a fireplace in the dark world. I think one of the locations is the burned slide reel room which I already found, but the other in the hidden gorge I didn't get a chance to check out yet.
Speaking of which, the slide in the burned slide reel room seemed to indicate that there was a light-warp thing hidden in the rock as you sail around on the raft, but I went around the raft route in the dark world, shining lights on the rock, and found nothing. Annoying
The first combination lock that was in the main area didn't seem to give me a whole lot of info beyond another diagram showing a red x. Perhaps I missed something.