"Many many of the fans will need this person's autograph to complete their collection"
For this reason, I don't think that it's Daisy Ridley. I imagine that a lot of people will still need John Boyega, Oscar Isaac, Adam Driver etc as they haven't done conventions or signings either, so meeting Daisy wouldn't complete the collection. It won't be someone from the prequel trilogy either for the same reason.
Harrison, on the other hand, makes more sense, as Carrie and Mark have done conventions, along with the other main original Star Wars cast.
It has to be someone where the rest of the main cast have done events.
I don't think it can be a main Harry Potter cast member either because too many of them haven't done conventions or signings.
"and this will most likely be their only event that they will attend" also makes me think it's someone older. If it was a younger actor, how would they know that they'd never want to attend an event in the future (eg Daisy Ridley).