Poor writing really. A mystery writer doesn't add facts that are required to solve the mystery right at the very end. "Oh, by the way, this is George. Nobody knew anything about him but he was in the room where the murder occurred."
I wouldn't call it poor writing honestly.
Most of the mystery stories are usually about actively investigating to discover the truth and told from one of the main characters' POV, for example: Detective Conan with Conan/Kudo's POV at most, or 12 Suicidal Children with the POV from most of the casts.
This author however, for this manga, seems to put the readers in the 3rd person respective. We're neither the main protagonists or antagonists, we don't follow them actively investigate things, we don't get to see the full flow of thought of the characters reacting to the situation, we're like people just being there to witness how thing happens and unfolds most of the time.
And because of that, most of the hints can only be noticed after the facts, which give such a interesting re-read, like for this arc, chapter 34 was Nezumi's first appearing, and at the end of that same chapter, we meet the grandma, and now, we know they're the same person. Or chapter 35 mentions about the Tsugumi house was supposed to only have the old man and woman at the time of the incident, but in chapter 37, the old woman was not mentioned in the list of the victims, which again gave another hint at the grandma and finally confirmed in chapter 38, which was released 2 weeks after that (following the raw release in Japan).
Another interesting detail is the mention of Keyser Soze in chapter 34. Soze is the main mystery in The Usual Suspect movie and his identity is only revealed at the end of the film, which is kinda similar to how this author doing the mystery in this manga like I said above.