First, I wonder if maybe a reason that some people get a strong sense that the parents feel guilty or have an idea of what happened to Isa, while others feel just as strongly that they're innocent, is because both are true. Eg., and this is completely imaginary, maybe they suspect that a sports coach took her, because in hindsight they remember instances where he was paying her too much or the wrong kind of attention. They're the ones who signed her up for the team, so they feel guilty about placing her in danger by allowing this person into her life. ...but on the other hand, they've known Coach a really long time, and he's a good guy, etc., so they have doubts and maybe feel guilty for suspecting anything about him. And in this situation, they would kinda know, but not really know, what happened.
Second, about the 6:30 dogs barking/voices heard...usually when I hear these things at 6:30, I think, "Oh dear god, is it morning already?!" and try to fall back asleep. I wouldn't assume anything was amiss. I also have had the experience of being half asleep/half awake and not being 100% sure what is real and what is a dream re: noises. I often experience lucid dreams (ie I know when I'm dreaming and can somewhat control it) but Ive noticed that when I'm semi-conscious because a noise woke me, in hindsight it is hard for me to say what was in the dream and what actually happened, and I'm very suggestible about it. Not saying this is what happened with the neighbor. I just can see myself being convinced that something my semi-conscious brain processed is much more significant than it really was if I later learned that something might've happened around that time.