bottom right drawer's 4-letter code. I assume the solution is a 4-letter word with the required letter-shift (11 letters ahead in te alphabet for each letter)

the medallion hanging from the beam above the desk

to open the drawer with the corresponding lock

5-by-5 matrix of red buttons

There is another four letter word visible somewhere in the room. Change one of its letters to G to get another four letter word

You got the cupboardwith the 5*5 matrix of red buttons on it, right side of the desk by the window. You can push the buttons to change the color red/black. The hint to solve it is the medallion hanging above the other desk. If you zoom on the medallion, you can see the pattern, even if you don't cut the medallion down.
Now. If you go back to the cupboard with the 5*5 button-matrix, you'll still be informed that you have no clue to solve it - because only when you cut the medallion down do you learn that you have to rotate the pattern 180 degrees - put it upside down, that is. BUT! Despite "having no clue to solve" the puzzle, you can still push the buttons. And, if you happen to somehow figure out that you have to rotate the pattern - like i did after trying lots of other tricks - the cupboard door will open! - you will actually see the door open behind the "You have no clue to solve this" sign :D In the cupboard you'll find the knob you need for the puzzle in the glass-door cupboard just right of the exit door. Even before you can open the glass door of it, you can see through the glass door that there are three knobs and one missing. Now, when I opened the glass door with a knife, the knob suddenly disappeared from my inventory - and I found it nowhere.
When I restarted the game and played it withoput that cheat, the know stayed in my inventory and so i could use it

Just besides the locked door, there is a plant on top of two wooden crates where you get one of the gems, there is a label " Handle with CARE" with a arrow sign below. The hint using the magnifying glass on the picture show R->G, means- change the letter R in the word "CARE" to letter G. It becomes CAGE.

Apparently the correct sequence is yellow, red, yellow, blue. However, C2 on the map is quite obviously green, NOT yellow. What on earth is the deal with that? I saw no clue to indicate that Green --> Yellow!