I'll have to check on the first part but I believe the creatures were only on one side of the building.

As for the snippet at the end, I'm sure there was a time lapse involved there. No way the Mallers hang around the area with so many biters and limited ammo supply. It's likely they came back after the creatures dispersed. The only question I have is why the Mallers could find anybody since the creatures ususally drag off any live/dead bodies with them?