Pietro is right, and this is something that also crossed my mind when there was talk before release of the bombing scene not making the final cut. Having extra "flashbacks" tacked onto the beginning wouldn't have been good for the structure of the film.
The editing was't a problem with the third act. The issue(s) there had more to do with the writing, or how the story unfolded from the time an Alien was detected aboard the ship until its demise. Basically they wake up, find dead people, and go straight to corralling it out of the airlock with a few close calls thrown in just to keep it from being completely ho-hum.