I’m trying to figure out the hints:
Mother could mean nearly anyone, although I am leaning towards either Evelyn Sader or Dot’s Mother.
Sugar Queen:
This seems out of the blue, but maybe Dot’s mother was, like, Arthur’s older sister, and Dot is the one true Queen. Alternatively, there might be an important ruler in one of the kingdoms that Agatha and Tedros run into who is a sugar queen
The wedding is probably Sophie’s and Japeth’s because we know that it is important to his becoming one true king.
The ex is another hard one. Maybe Rafal, as it is hinted that Japeth is named Rafal, and also maybe he did what the good school master did and put his soul into something and will come back.
The stalker, I have no idea on.