These are reasons I initially wanted to hate it but also reasons I think it was great in retrospect. He didn't go after the porcupine man at first because he was losing his grip and starting to be haunted by paranoia. If he had gone to the others with those large decorative porcupines as some kind of evidence they'd laugh him off. It wasn't until seeing the porcupine candies in context that he could make a definitive connection with the other evidence.

As for ending up living with the girl with the dress, I believe there was a part of him that was in fact trying to put himself in a situation where the killer would come to him. This is part of what pissed the mother off so much, and left him utterly speechless to defend himself. But his pride as a detective and his word to the last victim's parents meant so much to him that he was driven to do whatever he had to do. And his gut turned out to be right, but deus ex machina mercilessly punished him for that pride and served justice to the killer all in one fell swoop.