First off you should look at the game design through the eyes of the potential gamers and not through the eyes of game designer. This is helpful to avoid creating nasty traps, clever puzzels etc. which are just ignored by the player or which he finds annoying. Therefore here are the two cents of a potential player:
You lure the player into a trap here. The players find a weapon which is finally useless to some degree, at least he will be disappointed of having a weapon which is a better lock pick. Building up an expectations just to discover that it is worthless is really a lover killer. When building a new game world, you need to consider the knowledge of a new player, therefore the line between refreshing surprise and deep disappointment is really fine.
You'll get a gun, but it's absolutely useless...
You won't be able to see how many bullets you have left in the GUI
A boss fight... well,fighting a boss is more action game and kills the immersion of a horror game for me. Game like RE6 are no longer horror games for me, but this is just my personal opinion.
There will be boss battles, at the end of each area of the mansion you will be able to beat the boss.
Will this really be useful or scary at all ? I mean I never remember the placements of furnitures when running through a level. This could work in a story or movie where you give some hints and focus on this later on, but a game is always difficult. Either you need some cut-scenes to focus on the event (before-after) or it should be a very prominent furniture.
The games scares will be scripted, and not random. Things like furniture changing each time you go from room to room will be random, subtle things aren't included in this, but I mean when you enter a room for the first time.
Yeah, ghosts in mirrors are always scary *shudder*, so I like it, especially the interactive nature. But you need to be careful about the implementation of this scene. A new player would like to investigate the scene, either inspecting the mirror image or the roof. Waiting to long and getting killed for this is frustrating, especially if there aren't any hints about the danger, and learning by reloading isn't the best game design.
One scare for example, is when you go into the bathroom to wash your hands in order to progress, and you see a twisted little girl in the mirror crawling along the roof in a dress with her head twisted around, no eye sockets and a severed black jaw. A steady horror sound will begin to play, if you look at the roof outside of the mirror - you will see nothing. If you look in the mirror again, she will be behind you. At this point you need to leave the room IMMEDIATELY or she will kill you. When you re-enter there will no longer be a threat in the room, just a silence and the sound of Pier breathing.
This is a good example about the difference of a horror movie/story and a horror game. A horror movie benefits from the inability to control the character and the audience need to watch the upcoming danger with horror, pleading that the character turns around and sees it too. This is not really possible in an interactive video game where the character is directly controlled by the player. Cut scenes destroys the immersions often and the interactive nature of the character controls lessens the effect (e.g. the character looks at his hands while washing it, turns around too fast, opens the inventory). So, be careful to implement scary parts known from movies or books, they are not really useful.