I can help with the .303 tracer, when viewed at an angle and side on, the rounds tend to look like individual dots. It's only when looking from behind that you get a view of an apparent tail on the rounds. Youtube should throw up some Gulf war footage of Iraqi air defence firing tracer that will give you a better understanding.
Without the airbursts wouldn’t it be hard to portray what’s happening on canvas? Would be interesting to know if the Flak guns on the dam could depress their guns far enough to be of any use during the run in to the target as depicted during the film and in various paintings. Having walked across the Mohne there are walls either side of the walkways and I doubt that they were positioned to cover the surface of the reservoir when the aerial threat would have been perceived to have come from above. Obviously when the Bombers flew smack bang over the structure they became sitting ducks.