I'm not really sure there would be a good honest way to reveal Rocco's treason if that were to in fact occur. It would be hard to write such a scene into the film and keep it from coming off as pedestrian and in-your face. Rocco didn't really talk to other people and a scene where Roth and Ola approach Rocco personally would just be to obvious and clumsy- especially since it would have had to have happened before the start of the film.

It also would have muddied the waters for the viewers of who was the traitor. So even though it fits beautifully on paper, it really would have been a difficult reveal on film. As it is, i think the subtlety of it works just fine- even though many don't have any reason to believe it. (and isn't that how Rocco got away with it?)