Why did no one bat an eye at Sunset and Pinkie on stage with PostCrush? They're also band members, so everyone probably assumed they were surprise guest stars.
How is Sunset not affected by the reset? She was far enough from the festival grounds when Postcrush activated the Time Twirler for the first time. While it didn’t save her from being trapped in the first place, it was far enough to let her retain her memories.
Despite Sunset's earlier insistence that Pinkie shouldn't be wasting time on her other antics, they make it to the concert with plenty of time to spare in the final loop. It's possible that Pinkie didn't cost nearly as much time on her own as Sunset did trying to stop her, so time was saved by playing along. There's also the fact that they won a two-seater bike in the final loop which almost certainly cut their travel time.
How the Time Twirler works or who it effects are never explained. Maybe Sunset Shimmer wasn't the only one who was stuck in a "Groundhog Day" Loop, maybe everyone who was in the blast radius Sunset saw was stuck in a loop. The other Mane Seven might have figured out Equestrian magic was at work and eventually figured out a way to stop it, but everyone else at the concert would have no idea what was happening to them.
If the only thing keeping the Dazzlings under control was that they were unaware that Equestrian magic was loose in the world, then Sunset Shimmer telling them about the Time Twirler might get them up to their old tricks.
Fortunately, the day reset one final time between that moment and the destruction of the Time Twirler, so odds are they won't remember that conversation. Of course, if there's still magic loose and other artifacts unaccounted for, the possibility remains that they could find out on their own.
