i feel that they change the backstory after they already made the scene, they probably use the discarded material to make those trailers. in the movie, the mothership is non-functional and not operated, all to add the sense that the alien are less intrusive and at their mercy. apparently they are that dumb. [rolleyes] which is why the guy was surprised that Christopher show greater intelligence. maybe they have a social caste and only a handful actually know what they are doing.
[Spoilers!] One of the people "interviewed" in the movie said that they seemed to be the workers of the race and that the higher up's all died off. This accounts for their seeming lack of greater intelligence or initiative to rise up. Christopher must have been a pilot or navigator of sorts (he knew how to pilot the ship!) so was in a different class than the others hence the greater level of intelligence.
The Prawns are a 'hive' race. A few 'queen'-like entities(we're never shown them, i'm sure we'll see them in the sequel) control the thousands of 'worker drones', going from planet to planet and harvesting their resources so they can sustain life on their own. A disease killed off all the queens, but the worker drones were more resilient to infection(due to their very nature of life) and stayed alive. The mothership went into auto-pilot after the queens died and took them to the nearest planet that could sustain life(earth). The practically brainless drones stood in the mothership for months(years?) until the humans finally broke in and took them out.