In fact, this is a big impossibility, not just a "litle thing with no importance". The argument of "controlling the dream towards a second and a third level" would be aceptable if a one person dream. We are talking about 7 persons that share this diferent levels - to much of a distortion. The natural way to go would be each one of them walk their unique way since the second dream level. I cant acept this size of divergence.
Having said that, I close my eyes to that because I loved the movie. What I loved about the movie was the exploration of the mind features witch I think to be accurate.
Sorry for the english, it not my natural language.