The camera and photo it produces is used in all but the soulless route. You need it to get one of the books with the help of the Shoe Ghost.
The photo from the fireplace is another photo used to get a book, i dunno what you mean about them being unused?
The item in the right side garden only becomes available after getting an ending to the game.
Thanks for the answer : ), maybe my translation was wrong or my expression was wrong. What I'm trying to convey is not that these things are useless, but that I found bugs (maybe?)at what time. I'm not stuck in a scene, I know where these props come in.
What I'm puzzled about is: seeing the MC's reaction I feel like she should have met Diana, but in my memory the MC never seemed to have touched Diana before, so I wondered if I missed a scene with her, or if Diana appeared early,