You can find it on the wall-mounted key holder by the desk around which the sewer map was found.
The other key (to be used around the B1 exit) is found in the water at the other end of the location where you use the key.

You cannot find this key unless you move the shelf and find the crack in the floor.
To obtain this, you can use a magnet and thread, which can be found on the floor in the motel office.

It has nothing to do with the order of the signs in the horoscope.
The numbers simply indicate the number of limbs.

You can obtain a camera in the final stage in an area that is equivalent to the 2nd floor of the hospital in the real world.
If you read the camera instructions, you will see that a flash is included.
The titles of the pictures hanging on either side of the altar are, "Light Illuminating the Dark" and "Light of The Future".
Here, use the camera and flash in front of the pictures.
You should then see a figure showing the alignment of the buttons.

Obtain the "Ring of Contract" at the jewelry store in the final stage.
Before you pull Melchior's Dagger out of the fridge, use the ring to join it to the chain.
This hinders the monster, which is sealed inside, from coming out.

If you can stay clear for 30 seconds in real time, the monster disappears by itself.
If you still have bullets, however, it does not disappear.