Ah well, like I said, it's probably the only explanation for it anyway. Besides, not everyone's gonna dissect the trailer like we do.
Then again, I don't know anyone else who's gonna watch the movie. I'm hoping to be surprised by the casual movie-going public.
Part of me agrees with you on that last bit, though - to have a total xeno attack at the end where it's even the main conflict would kind of be like how the film Sunshine fell from the pedestal it set itself on with its first two acts. Fortunately the last five minutes or so were absolutely brilliant - or enough so that it picked back up, at least. Hopefully the case is a better one with Prometheus.
Edit: Okay, so apparently Noomi Rapace is being reported as a "female Space Jockey" in
this article. Um...
what?