When Jenny hands the garment bag (asupposedly holding her after party dress) to the assistant, it is flat. When Serena is wearing the dress that was supposed to be inside it, it has an extremely poufy skirt. The bag would not have been flat while holding the dress.
Serena and the socialites were all sitting in the first row when they were called to replace the absentee models. The seats are still full when they strut the runway. As it was a ticketed event with assigned seating, it is not explained who took their place to fill the empty seats.
Blair and Serena's "tradition" during fashion week is to watch Eleanor Waldorf's show backstage. But at this point last season Blair and Serena were not speaking to each other, and the year before that Serena was away at boarding school. Meaning they haven't observed this "tradition" for at least three years.