Just finished the game. I think D had something to do with Brian's death in someway shape or form. There was a hint that Brian was attracted to her and when you finally get to her tower there's a book on her bed that talks about "leading someone on and betraying them". In the big climax of the story that's pretty much the only thing you find in her tower.
She creates false police reports about the girls and doesn't tell anyone about Brian. She has an interest in keeping things silent and then she bails right away the first second she can.
She tells Henry she snuck in the middle of the night to plant the private two way radio so she could do the same for starting the fire during the radio technology.
I don't have it all planned out and the whole thing is all red herrings. I don't know if the game is interesting enough to even figure out but I think I'm on to something with D being the antagonist. What do you think gaf? Do you inspect the book at the tower?
Being as how Brian was 12 and Delilah was around 40 at that time, I don't think that romance part of your theory is very accurate.