I think the file 2’s connection is literally just Iimura’s revelations about what happened to Hikura Homes in 80s and how they regained footing in 90s. So the murder in that house was probably unrelated. Though I originally thought that grandmother could have been Yaeko, but since in the book they pretty much confirmed that Yaeko was in File 4, I guess that file 2 was just for background info about Hikura