About room 12..
...could someone explain to me please why the diamond wall isn't reset at each iteration too, allowing the doctor to do what he does ?
The way that I see it, or interpret the episode is that the doctor dying in each iteration was only metaphorical, in his head. Each time the doctor tells the truth, the interrogation machine was one step closer to releasing the doctor, but the room where he fails the interrogation doesn't fit the dynamics of the castle, which is meant to torture him, so everything resets in the castle except the room.
As a form of torture, it forces him to replay the sequence of events over and over until they get the answer of the "hybrid." Since he "dies" in the same room over and over, I imagine that this sequence stands still and doesn't change, because the the Time Lords wanted the doctor to reveal the final truth in that room with the diamond exit.
At least, that is how I see it. I am clearly in the minority since no one else has such a crazy theory.