I don't know about an in-story justification, but plot-wise, Cadance was taking Spike's position of trying to get Twilight to calm down. The difference being that Twilight actually listened to Cadance.
That was a pretty basic breathing technique. I wonder why Twilight hadn't picked one up, or at least used one, before. Maybe she only tries to calm down when her old babysitter makes her? Still, that really made Twilight look like she was being treated as child. It makes sense given their relationship, but it also highlighted Twilight's immaturity to me.
I don't know about you but I liked seeing this side of their relationship. Cadance is fun and all, but she's still an authority figure. She is in a position to treat Twilight like a child.
Making a big event like that go according to plan involves busting your hump and dealing with a thousand unforeseen events. Just showing the dirty work involved it making it go off would have plenty of entertainment value.
Works for me. I love seeing the work that takes place behind the scenes of big events. What happens backstage is just as interesting as what happens onstage to me.
I think the idea is that the false inspector didn't do anything that the inspector is supposed to do. She went way off script, forcing the manes to go way off script as well, and they managed to improvise.
It was a clumsy resolution to be sure.
I think that it would have worked better if they had shown random crystal ponies treating Mistress Harshwhinny well, instead of having her being ignored.
That would have been a much better story. It would hae even been heartwarming to see the crystal ponies earn this victory for themselves.