Very good discussion topic. These stories make a very hard target to contain an active shooter. Not only are the isles and displays tough, but they have numerous backrooms and storage areas in the rear. The toughest training I ever had was responding to an active shooter at a large high school. It is UNREAL how many "pods" and long hallways there are. If you ever get the chance watch the special (I think it was NBC, not sure) on the Columbine response, it's really an eye opener. Luckily most jurisdictions have changed there method of responding to these calls. I can't go into it because of OPSEC type of stuff.
Back on topic- when you respond to one of those large department type stores, the store layout is one hurdle to overcome. The crowds inside that are scared are just as big a hurdle. Each situation is different from the aspect of LE tactical response. However, I personally think that a CC or OC person in civilian clothes already inside the store is the best hope for a quick resolution. There really isn't a great place for cover in most buildings, just concealment. I think straight up surprise is the best option (ie-CC'ing plainclothes).