Ok, so I did some research on the exoskeleton issue, and I found some contradictory information...
I did find several sources which state "exoskeleton" or "carapace" which is the dorsal side of an exoskeleton. However, those sources were primarily fan-speculation, and unreliable sources. Ultimately, I think it depends on what 'you' consider to be cannon (aliens colonial marines, anyone?).
I'm not the type to suddenly change my mind about what I consider to be cannon just because it coincides with my perceptions in a debate, and unfortunately I don't consider one of the primary "exoskeleton" sources to be cannon (A:CM, anyone?). With that being said, I'm going to swallow my pride a little bit and admit that the alien was not originally intended to have an exoskeleton. I found interviews from Ridley Scott and H. R. Giger which do specifically say that they wanted the Alien's skin to be translucent to the point that you could see its skeletal structure, circulatory system and organs. However, due to the limitations in special effects at the time, Ridley decided to go with the rubber suit instead which happens to appear more exoskeletal. Other contributors to the franchise probably just assumed it was an exoskeleton also and ran with it (A:CM, anyone?).
But going back to the point I was originally trying to make, the alien's skeletal structure, whether endo- or Exo-, is still very rigid and covers a greater portion of its torso than ours (stomach and kidney areas) which would make it less flexible than ours. So I still don't think the alien's rib cage would expand/contract as it is breathing. But perhaps that's just a visual artifact of the rubber suit too. But again, it's all just speculation and a possible explanation as to why the alien dropped off the motion tracker when it stopped moving but not Dallas. Another possibility, just as likely but also speculatory, is that Dallas had some sort of locator chip in his pocket or something that the motion tracker was also reading in order to see his location while at a stand still...
As for the PC version of the game... It's probably just an update and we'll see it back up soon. We've seen that sort of thing with many games in the past (A:CM, anyone?).