@ Fischboy
Purson himself wasn't relevant to the plot until now. Why was he foreshadowed? Maybe because it would be an asspull to add a new character out of the blue? It's almost as if the author understands these things need set up.
There's a simple resolution to this, he stops reading when he hears the trumpet. "Oh Amelie, do you hear that? That's the pixie, I guess story time is over for the day." Did you see what I did there? I integrated foreshadowing without taking away from the subject of the scene. He only had to do this once.
You have it fundamentally backwards. It's because it was directly related to the plot now that it should have been mentioned earlier. If Iruma showed up next chapter with Hell's Tuba, the most powerful and beautiful music instrument in the underworld that will guarantee make their musical performance a success, would you find that an asspull? Why? Is it because there was no mention of it previous?
You're arguing it was impossible for the author to foreshadow the pixie but not for him to foreshadow Purson himself? That's pretty self-contradictory.
