I just thought of something but what if in the preview the reason why Ash,Pikachu, and Clemont are so shocked during the fashion show is not so much Valeria coming out on the stage but because Serena, Braxien,and Bonnie come out on stage and wind up being in the fashion show. Maybe the other 2 characters that we see in the preview that look like they could be up to no good do what some people have suggested they do which is that they might "Kidnap" Serena, Braxien,and Bonnie(I think there were some other people that suggested the whole "kidnapping idea" so that they can have them be in the fashion show.
Also the reason why I say this is because Ash,Pikachu, and Clemont seem to be at the fashion show watching so while they might be surprised to see Valeria come out on stage during the fashion show(Although the main problem with this idea that their shocked that Valeria came out on stage is that in the summary it indicates that they would already know that Valeria is in the fashion show since she told them.) although I would think they would be even more surprised to se Serena, Braxien, and Bonnie come out on stage. Also another idea if that does happen is I think it would be kind of funny if when Bonnie comes out if she does her whole marriage gag on Valeria. Now keep in mind this is just all an Idea and I'm not saying that it will definitely happen but that it's just one possibility that could happen.
Plus it would explain why Serena and Bonnie disappear and why Ash,Pikachu, and Clemont would be shocked. Also the battle between Shota and Valeria where they use Bagon and Spritzee respectively probably happens towards the end of the fashion show and then afterwards would be when Shota meets Ash and the others and has a double battle with Ash.