after 5am, probably in some dawn light. Is this an indication that she went over to the windo herself, willingly
That's always been my interpretation. I haven't read anywhere there were footprints THROUGH the house to her room, in which case not only is it more likely even 1 person in a full house would stir, let alone the dog alert. I think the abductor came TO the window, possibly lost an already loose red thread opening it, and spoke or even pantomimed to her. Maybe she was up watching cartoons by herself, or the old pebbles on the bedroom window summons, who knows. Bundy was very smart, as well as clever, and it's possible he had shown her something 'neato' before, so in him beckoning her out she was anticipating a litter of kittens or something. If she knew AND trusted him, no ruse needed.
I don't see mention of the front door being printed, but she may have unlocked and opened the door and met him outside. Incidentally, not all dogs go bonkers with this sort of thing. If the dog was awake, in the room, and watching it wouldn't be unusual for him to see her open the door herself. And dogs don't wear watches, so he might not be concerned at the hour. Or he WAS right there, Bundy patted him on the head from the threshold and they left.
Whether the Burrs were on Bundy's route or not doesn't mean he didn't 1) know her to speak to or 2) recognize her from passing by while she was playing outside, etc.(thereby she might recognize him). Sidenote, I'd be interested where his paper depot was, and whether he worked the next day or was sick. Based on the timeline, he might have been out and about already, he must have been used to getting up early. He clearly was able to compartmentalize, so he could have killed her and shown up an hour later for work.
Following the theory of he knew her, she'd likely ride on his handle bars, or if he'd rode over in a deluge (which wouldn't deter an obsession or paper route) and stashed the bike, she'd at least walk with him. We can't forget, he lived and likely rode through the area...he'd be familiar with wells, drainage pipes, abandoned sheds, etc. But based on his future planning, he probably had an idea where he'd put a body. From what we know of him, he seemed to have a woods preference. If her body was in the woods but not concealed, he could have come back w/ a shove. I don't know if a foundation would be his first choice, but then again if it was one of his first murders, he may have tweaked his MO. I believe even at 14, he would have had at least some forethought. He also would have been charming, and 8 yr old girls have crushes that parents don't know about.
As for the letter to Mrs. Burr, he didn't have compassion, but I think there was a wisp of regret. Not for the victims necessarily, but in that he understood on some level it was wrong. It's also possible AM's murder was not quick, or particularly brutal. Maybe he was fascinated and came back to the body. All of which he didn't want to dwell on in his own mind, or be questioned further if he admitted it. Which makes me wonder if her remains are somewhere recoverable. It's unclear if him speaking of himself in the 3rd person was just a facade, but he obviously had some writes crossed, and knew it. I too believe there are other child victims, he seemed to mix a plan (crutches, missing passenger seat) with sheer opportunity.