for the face hugger, let just say that it was romaing around in the atmosphere processor, somehow jumped on the dropship. as it was outside of it, it had to face air-free environment, etc. We know from the first movie face huggers can survive extremely low temperature, w/o available oxygen, for quite some time. But that doesn't mean they can run around easily while trying not to choke. So while Ripley and the Queen are playing together, the face huggers are, well, simply sleeping. They wake up later, start searching, find the crew in hypersleep.
Additional note: the facehugger implants Newt, not Ripley, originaly. I dunnot know if the scene will be featured in the director's cut movie or has even be shot, but it was scripted: when Newt capsule is flooded with water, drowning Newt, the queen escapes Newt and goes into Ripley ( hugh! ).