I guess room for natural action and instinct isn't nessesary for the Xenomorph. FHs are expendable, theirs always way more eggs then needed. The active ones seek new hosts. Why do you think there were some running around without being disturbed. They didn't activate in the first movie because they were in a stasis field on the giant aliens ship remember? As soon as the field was breached one came up, and he was taken immeadiatly, No need for new ones. The FHs wouldn't know that they were burried under the ice.
[spoiler]How do you think their were a thousand drones in the flashback, their were only 7 places for chosen sacrafices. But the queen always makes more face huggers, you saw that their was a city built around the pyramid. In the books the drones devide up into castes, some gather hosts, some attend to the queen, others attend to the eggs. Face huggers are free moveing as well though, they seek out hosts on their own. I would theorize its because they only live so long egg or not. I would simply sugest you read a book[/spoiler]
As far as charicter development, a charicter changeing from one thing to another through the coarse of the movie is the very definition of charicter development, the subject in question goes from being a frantic mess after confrontation [spoiler] then fighting along side the predator, then stabbing the queen who is really really scary and mean. That charicter developed a whole bunch, just watch the movie [/spoiler]