I don't watch Corrie but here are a few thoughts anyway:
1. Back in my student days I was on my way into college when I came across a crowd of people who were watching a fire in a disused warehouse. It had hardly taken hold when I arrived (and, of course, joined the rapidly-growing throng) but it was a raging inferno, with the roof falling in, about 10 to 15 minutes later when I asked "Er, has anyone here actually called the fire brigade?". It turned out that everyone had assumed that someone else must have done it! (So perhaps that might explain the Corrie scenario?)
2. TV dramas seem to consistently deny any knowledge of first aid procedures. For example, a cop will get shot and, instead of trying to stem the flow of blood, his mate will always lean over him simply to hear his dying words (which are guaranteed to be something poignant, rather than "Call an effin ambulance, you tw&t!").
3. (Re: "then they all watched Leanne go back in"). Further to my example above, people seem to assume that someone else must have taken charge. (i.e. "If she's going in there she must know what she's doing, so why should I stop her?").
4. (Re: "how come Carla was so badly affected by the smoke so quickly"). Er, asthma, perhaps?
Carefully analysing the plots of TV dramas is probably never a good idea nyway. I once sat next to a guy on Eurostar, who was studying the script of a TV drama. Being naturally nosy, I was reading it over his shoulder and trying not to laugh out loud at how utterly preposterous the whole thing was; I assumed that he was a college lecturer reading a rather poor student submission. It was only when he closed the file and I could see that the name on the front of it matched that on his ticket that I realised that he was the director of The Bill, planning shots for a forthcoming episode ;-)