Technically, this would be the largest standard Alien animated in the media, so maybe the louder footsteps are in line with the size? Like how the Queen sounds? That part in the movie where it's in the wall could be due to how it's full at that point, and just finding a place to snooze. As for breathing, it might or might not require to short term, but the dynamics of smell requires it to inhale the scent of prey. That is, if they hunt by smell, of course.