Just take the parts of my advice that's useful (if any). The dialogue I wrote is just a way of showing what I suggest.
I understand what you're saying but I want the question to be the driving force and adding in some of that detail in conversation means some of that mystery needs to be mentioned. Keeping the dialogue basic means I can imply Arthur has lost it while holding back detail. If I begin a deeper conversation in the interrogation room then it wouldn't feel natural. People will read it and wonder why key things weren't mentioned. It would come over as a trick rather than natural conversation.