Aww man, late to the party. Guess I'll just issue my assumptions and move on:
In Alien I believe the Xenomorph was watching what she'd do. We know enough to know they can hide and come out at opportune times so why now wait it out and then make your move? They don't have eyes anyway so aside from some other sense we don't know about it might not have known what she was about to do when she flushed it out. That's my take at least, I could be wrong. That's where having to rely on he said she said makes it confusing.
As for the whole bug hunt comment it's clear the Marines have fought Xenomorphs before but nothing of this scope or scale. More likely like what was said above they were sent in to exterminate some native alien species that before or during human occupation of an alien planet or moon. The reason they were so easily defeated is set up early in the film: their arrogance and individuality, something Cameron touched on when he said he wanted them to resemble a marine unit fighting in Vietnam. While the results might have still been the same if a full fledged tactical marine unit modeled after current day units had been sent in I highly doubt they would have been defeated so easily. They basically had no plan of attack aside from "kick ass and take names". They obviously weren't trained well enough to do the job but a darker reasoning for this might be because The Company knew full well who they were and knew what the results would be. After all the goal was to bring hosts home that had been impregnated. Why train the best if that meant no surviving experiments to use once they got back to Earth?
As for the life cycle question it's hard to say. They seem eusocial like Wasps but the queen is the ultimate authority and with so little to go from it's hard to gauge whether any other hives behave in such a way. Since they take on traits of the host they inpregnate their social structure could change from hive to hive. More likely is the fact that very few soldiers are created and they're kept in check by how large the host population is (hence so many in Aliens yet so few in Alien 3) and other environmental conditions. Their lifespan could be as long as it takes to spread their eggs or they could live for lifespans approaching humans. We don't know since again there's little officially to back any reasonable assumption. That's OK though it keeps them mysterious and I like it.
I will say that I don't really care to include the AvP films as canon as to me they're a disgrace to the franchise. That's my .02 at least.