As for the infestation idea - what about this: if a dretch would kill a human by headbite, and alien team has got stage two at least, he would infest the human instead of killing him.
Now, it counts normally towards his kill count, but he has exactly 60 seconds to do something with his new infested human : after first fourty, the suicide timer will start.
As far as I can tell, this would mean several things:
Firstly, the spawnkilling problem is solved, as the infested one hasn't a chance to build a telenode in a powerless location such as alien base, unless the humans let two or more builders be stolen away from base and not hunted down, in which case they clearly deserve it.
Secondly, it would make the "useless defences" discussion less critical, since it takes time to reach the base and with the alien forced to act quickly, far less time would be available to destroy defences, thus making it not nearly as important, unless the humans were already under siege.
And, it seems only fair, since humans can be infiltrated, what about making a brain control secondary mode for some weapon that would work only on aliens in "near death" health zone?