There is an item called the Sticky Frosting Scroll which I believe was created around the time Elsie was made as a challenger in the BC; however there is no information about the scroll on SubetaLodge, and the item information for the scroll just says ‘opponent scroll’… is this just an error created by staff so that the item is actually unobtainable/ doesn’t exist?

Opponent scroll means it's a scroll used by a challenger in battle. Generally you can't win or create those.

Ah okay that makes sense! Are all the other challenger scrolls on the item directory then?

Unless they are set to hidden. Occasional they do that if it's supposed to be secret, like before a plot or event begins.