So i believe the imposters true goal is to save the earth from the crewmates. Now my really dumb theory is that the crewmates have gone mad during their space exploration and they now need to go back to earth, but the imposters/imposter heard them planning to start a killing spree once they land. That can also explain why the crewmates can throw another Crewmate off the spaceship without care.

The game sets place in space to test if the crewmates could finish basic task, teach the baby crewmates, and make it back.

The baby crewmates are little space creatures that can easily learn things. The Crewmates were sent to space to see if other astronauts could take them on space exploration.