What exactly was the relationship between Katsuragi Shin and Nishizono Yui? He claimed to have met her only briefly, but if you ask the right questions you can find out that he, NOT her brother or her semi-boyfriend, accompanied her to her abortion. Even if you try to challenge him with this, you can't get any more information out of him, though. If he and Yui were closely involved in some way, that raises a horde of other questions about Shin's involvement, or lack thereof, in the first set of murders. Did he know what had happened to her? Did he care?
I assumed he was responsible for her murder. She was missing all of her limbs, which fits Katsuragi's MO better than it does Kusaka's.
Why on earth was Mizuhara Mio assisting in the disposal of dead bodies? The game never even vaguely tries to explain this one. Obviously some latent semi-parental guilt is involved, but even after they met up again, it takes more than that to explain "By the way, I'm now a murderer, would you kindly get rid of these corpses for me?"