Regarding the killer moving around the house—do we know that the house WAS dark? Some people are big on night lights, and it might be possible that the kids living there weren’t the best at turning off lights when they left the room, either—many people aren’t.
I already replied to this question to someone else.
No, we don't know, of course.
But.
There were various pictures published from the house mostly its windows, few of which have no blinds/curtains on.
( I believe I do not have to go and find all these images, this topic was discussed in details and many images circulated. If I have to, I will. )
So, If the lights were on, and/or intruder used a headlight or torch, then an innocent person/dog walker, walking by the street or neighbourhood, would have been altered by what was going on in the house looking thru its unobscured windows and could have altered the police. Or it could have been a neighbourgh, suffering from insomnia.
By the way, it happened to me once. I was up late at night, studying for my uni exam. So, I do not know why, but I looked thru the window and spotted a torch light moving in the building opposite my windows. There were large modern offices and it was unusual at 2 am someone with torch to move around. I contacted the police straight away, they arrived in 3 mins, I was still on the phone with the dispatcher and the force arrested burglars trying to steal computers.
So, based on logic and probabilities, and let me sress again - this is JMO, taking into consideration the level of preparation and planning by the killer, there were minimum if at all interior lights and minimum using of torch, at least at the communal areas of the house, especially the ones with unobscured windows.
I still may be wrong, of course. Only time will tell.