I loved the scene in the forest where Benoit and the priest were debating and, unseen by them, but clearly visible to the audience, we have a figure walk by in the background with a lantern.
I very much appreciate how Johnson is committed to playing fair with the audience regarding the mystery, even signaling things like "you should be wondering if there's more than one culprit."
Even Agatha Christie sometimes cheated a little bit. (Solving Death on the Nile requires making your own map of the ship at home, along with a time table. Yes, technically you could do it, but that feels a bit hostile to the reader. At least put the map in the book, Agatha!)