This is just one of those inconsistency things that bothers you. When all the people were in the Sept of Baelor for the trial and then they sensed something was wrong and then got blown up. Why would the sparrow guards try to prevent everyone from leaving? They were just the audience, they weren't on trial and did not need to be there.
The High Sparrow was arrogant and didn't think there was the possibility she wouldn't show. He was informed she hadn't left the Red Keep so he sent a group of Warrior's Sons to retrieve her, so why have a panic and possibly show weakness by letting everyone leave when she obviously is going to be dragged through the doors any second.
And even if she isn't brought forth she would be tried anyway.
HIGH SPARROW: If the accused is not here, she will be tried regardless. We cannot escape the justice of the gods--
If she wasn't going to show the trial would begin without her, still needing the witnesses and audience. If the trial was postponed it would be showing that Cersei is above the court and can avoid punishment by hiding away, diminishing the power and authority that The High Sparrow has gained.