Well, I managed to have a look inside the door panel. After picking up my car, I noticed a distinct rattling sound. It turned out that one of the metal tabs that holds the leather inset had broken. My mechanic fixed the problem and allowed me to observe the procedure.

This also allowed me to query him about the fix to the door lock while he had the panel off. The relevant point is that the door lock mechanism needs to be removed to discover and diagnose the problem. This is because its workings are obscured by the door itself. Without taking it off there's no way to see what the problem is (at least in my case).