Re: RSVP Natasha: She'll be there, but things will hit the fan when she sees Shadow-Clint, especially when he attacks her. There might be some Vormir flashbacks there. She might or might not get injured in the fight, but she will definitely be rattled afterward.
Harry: Harry will be first to right when the Dark Wizards come in the room. He'll stick close to his friends if they're there, but he's also going to do everything he can to defend civilians in the middle. Afterward, I imagine he'll be one on the team to investigate this kind of magic. He might or might not see Snape or Sirius here.
Thirteen: She'll do what she can to rescue people- since she's good with science and still learning about magic. What will surprise her might be a glimpse of Missy or Rose. But she won't be forced to confront them (she will be concentrating too much on helping the others).