Just guessing of course, but to answer those questions, my theory would be this:
The two surviving occupants of the house were on the ground floor, I believe. Perhaps entrance to that is from the side of the house you often see in photos and the entrance to the other floors from a different access point around the back.
The perp knew this and went in and out that way. He confronted one pair of victims, gagged them and proceeded to kill them. A certain amount of muffled noise would be heard both downstairs and in the room of the two other victims. It might have sounded within the limits of “fooling around” noise given that it was muffled. So nobody gets up to see what’s happening. The same sequence plays out with the second pair of victims. Downstairs, the two survivors are thinking, what’s going on up there? But they figure it is either okay or too dangerous to intervene. Somebody’s looking out the window and they spot the perp leaving. He might not have seen them watching him, or he may for whatever reason think he’s done enough and spares them.
Despite all this, the two survivors don’t check out anything further until the (late) morning at which time they discover evidence of the crime and call in friends and the police.
As to 4:00 a.m. and not 3:30 a.m., that might be within the boundaries of (a) faulty time estimates either way, or (b) a certain amount of time the perp then spent at the crime scene rather than making a fast exit, although you would think (not being a crazed killer ourselves) that having done all that you would figure the police could be on the way because who knows what those other occupants or neighbors might have heard? I do wonder if the perp knew there were two more students in that other part of the house. He might have been so fixated on his victims that he was never aware of the third part of the house and thought it was a basement.
“ Despite all this, the two survivors don’t check out anything further until the (late) morning at which time they discover evidence of the crime and call in friends and the police”
No - one of the surviving roommates (Dylan) saw the masked killer.
She thought Kaylee was playing with the dog. The dog barked. She heard cries. She heard the murder say “I’m here to help you”. He walked by her!
You would think one of them would be screaming their head off or running.
Original story had both roommates on the first floor.
However now the truth is out that one of them had a room on the second floor and saw the perp. wearing a mask before 5AM.
911 call not made until around noon.