Also, I imagine investigating intermittent problems like this with new engines of this complexity is a bit like debugging a black box. They followed a process of elimination and picked the wrong culprit twice, but at least they didn't give up (I know what that feels like from experience with the S5).
1) In my experience, it is sufficient to let the car sit for 2-3 hours. However, the actual time will vary on ambient conditions (whereas I've had the rattle after letting the car sit for 2 hours in the parking garage at work, the dealership claims they need to let my car sit outside overnight in order to reproduce the problem). Mileage will vary.
3), 4) You needn't wait for the idle to go low. In fact, while that's normally a good idea, it takes a minute away from the failure window. You can slowly rev the engine immediately after ignition, and will likely first hear the rattle well below 2200 RPM. It moves up in the rev range after that and eventually disappears around 3000 RPM (it did for me, anyhow).