I have heard the devs mention in several game videos that enemy encounters will play out differently every time a level is re-played, including hostile numbers & the time & place they will attack from. So expect the action to be fairly unscripted.
The concept of having the aliens emerge from the "hive holes" in the walls from any direction around the player is an interesting concept, but it could end up like a game of whack-a-mole if not handled properly. We'll have to wait & see.