I learned from the audiobook of Rosemary and Rue that false leads don't have to be developed or specifically contradicted. They can simply be introduced, optionally with doubt ("I didn't think so and so did it") and the subject revisited rarely if ever. The investigator doesn't have to go looking for evidence they did it. They don't have to stumble on evidence suggesting that they did or that they didn't. They don't have to find conclusive proof the false lead didn't kill the victim. The false lead can just drop into the background, adding to general uncertainty.
Spoilers for and thoughts on some specifics of Rosemary and Rue's mystery below.
McGuire foreshadows Devon's guilt pretty clearly with the line about how Toby "couldn't tell if [he] was laughing or crying." (Paraphrasing, maybe.) The book suggests only two alternative possibilities in parallel, Raceline and the Queen of the Mists. When the Queen of the Mists is introduced, Toby notes she doesn't think the Queen did it. I think something similar happens with Raceline although I don't recall. It doesn't develop either: She neither pursues them nor finds any further evidence for or against them, other than Connor saying he doesn't think Raceline did it. The story works for me anyway.
It's a funny story, too, because half-paying attention I didn't notice any specific evidence Devon did it until the last few chapters. I noticed the foreshadowing and that he was just the obvious suspect and the one that was clearly foreshadowed, but never made the prediction. It seemed clear in retrospect, though, especially with no serious alternatives introduced before the half-way point.