I don't believe there is an incorrect letter indicated at 18dn; all present and correct here. If you genuinely believe that there is a problem, then I'd email the EV editor (email address published with the puzzle). I believe that he is normally helpful.
Challenging but fun, I think. Highlighting took a while as I was looking for the shape in the wrong way, but in the end I think the shape is quite appropriate for the author.
Artix is a fan of the cycling device in one form or another isn't he? Anyway, an interesting challenge and a surprising way of highlighting, made easier to spot given that it's part of my subject. I was pleased to be able to fill in some of the quote without needing to look it up. But enough of my being modest. Thanks Artix!
I thought this was a great puzzle. Lot of innovative ideas, particularly around the cycling of answers and method of finding extra letters and I enjoyed the subject and clever highlighting. Many thanks artix!