Final answer:
The disappearance of Violet Tillyman in a Floo Powder mishap is a fictional event from the Harry Potter series, with no specific year mentioned in the source material. Real-world historical records, such as the Commonwealth War Graves Commission referenced, are unrelated to the fictional events of the Harry Potter world.
Step-by-step explanation:
The inquiry about Violet Tillyman's disappearance refers to an event in the Harry Potter series, which is a work of fiction and therefore does not have historical accuracy. Within the context of this fictional world, Violet Tillyman's mishap with Floo Powder does not correspond to any specific recorded year, as her story is part of the additional world-building details provided by the author, J.K. Rowling. The information about Garnet's disappearance during a German poison gas attack in 1915 is unrelated to the question about the fictional Violet Tillyman's disappearance. As such, accurate details about the fictional event involving Floo Powder and Tillyman are not found within historical records or tied to real-world dates.