
Deliberate mistake: The Messerschmitts have painted yellow fronts, this wasn't done by the Germans until after Dunkirk. Christopher Nolan has admitted doing this deliberately so the audience could tell the difference between the Spitfire and the Messerschmitt during the combat scenes.
Answer: That's exactly how it was. If you search for images of the evacuation, the troops were stood in lines, waiting to be evacuated. The Germans were held at bay, and the air raids were periodic, so there was little risk on the beaches.