That seems to be the big mystery though - where the Alien is from. I still think it'll be the original one - even with the design changes. It's not like design changes were ever addressed (other than A3). Don't get me wrong, there's reasons as to why the design is different in some films but aside from A3, it was never addressed on film. Bit of retcon FTW.
As for it having God-mode, I really don't think they're saying it couldn't be killed at all. Just what the Nostromo crew (and you here) have available isn't enough. We never saw the flamethrower used against one of them so we don't really know how it'd effect them and the only time we see pistols do damage is close range (or explosive rounds from the looks of it in AR).