I've been following this pretty closely.
WRT the sole witness, Dylan Mortensen, as to where she was standing, the likely route of the killer as he left, why he did not see her, and why she waited so long, there's a lot out there with which to form some ideas.
Matching up her testimony from the probable cause affidavit (PCA), she heard three separate "events". She opened her door for the last two. The final event was hearing more crying from the direction of Xana Kernodle's room, she opened the door and saw the killer come toward her, walk past her, and head toward the sliding glass door toward the back, off of the kitchen.
Working from detailed layouts of the interior of the house now online, when the killer exited Kernodle's room, having killed the two people in it, and this after he had most likely first gone upstairs and killed Goncalves and Mogan, the route to leave caused him to walk, ostensibly in a darkened house, to a step up into a hallway, then turn to the right and go out the kitchen. Mortensen's room was at the hallway, to the left. He had to look down to see the step, then to the right to find the way out.
I'm going to guess that she had the door opened only slightly, and that unless he actually looked at the door fairly carefully, in the darkened hall the door would appear closed. He was probably with 3-4 feet of her, and likely still had the knife in his hand.
Seeing this, I'm going to guess that she quietly closed the door as soon as he had turned away from her to go to the kitchen, and she locked the door (she testified to this) and was afraid to make a sound for literally hours afterward, not knowing if the killer was still in the house. Think about it: if I open the door, it could be my death. If I make a sound, it could be my death. I'd almost suppose that she did nothing until the 1st floor roommate came up and knocked on her door late-midmorning.