I've read almost every post on these threads and have one question and one comment.
Was there anyone living in the first floor bedroom? Reports are of surviving roommates (plural). I have not seen anyone other than D mentioned.
Also, I am struck by how exposed M was due to the sliding glass doors. In the exterior shots, I see only flimsy drapes, or maybe none at all. It's hard to tell, but I expect that anyone in that room with the lights on in the evening was visible from the outside. Maybe it's because I'm a visual/observant type and always looking around when I'm on the sidewalk or in a car, but as a young person I took note of the fact that I was able to see into homes pretty easily at night. As I got older and more aware, I was always careful to pull shades down in the evening. I still yell at my husband about this. He says, "If they want to look at me, let 'em. Big thrill." Well, then of course we have the talk about the difference between how men live, and how women must live. It was eye-opening to him. The natural innocence of young people makes me sad when something like this happens. Bottom line is, situational awareness won't help you when someone is this evil.