Judging by the behaviour of the parents, particularly the mother, I really feel that the family (most likely the mother) was responsible. She just showed so little emotion. She had been through powerful chemotherapy drugs. She appeared slightly drugged up in later videos. Also, in order for such a young child to perform like that in pageants, it would require a lot of training and discipline (usually imposed from the parent). Most 5/6 year olds don't move or respond with such precision. It's as though she was a puppet and I truly wonder how much she actually enjoyed it. It all seemed imposed upon her. That raises a few red flags for me. Also, the mother referred to her at one point in episode 1 as 'the child'. 'why don't the police just go out and try to find the killer of the child?' or something similar. That struck me as very impersonal. I can't ever imagine referring to my own children in such a way.
The ransom letter was very peculiar. Definitely not a proper letter. It's not something that a paedophile would do....just abuse and murder a young girl, tie her up afterwards (why bother?) and then go and write a ransom letter. It looked blatantly made up. The handwriting looked a bit like a child's but the use of language was that of an adult and it was mostly spelt correctly, aside from one word. Also, how would a local paedo know John's name in the letter? That to me was a blatant attempt to deflect attention away from the family as though they had been specifically targeted.
The house was an absolute ramshackle of a place. Most people would get lost during the day time in there, let alone in the dark. Carrying a child down a spiral staircase whilst not making any noise or slipping would also be fairly difficult.
If she had been killed by blunt force trauma, so why also strangle her? Or why strangle her and then also hit her over the head? I think that she was hit over the head, then they staged the rest.
I also found the smashed window and suitcase part very odd as he said that he had smashed it himself months earlier when locked out and had come in that way, but "thought that it had been fixed". You would think that you would be more certain about that.
The house also looked very dingy. The basement was a tip and the train room was pretty bad. It just had a few signs to me dysfunction within the house. It wasn't a nice place for children to want to play.
I think that the Dad may have lied to keep his wife from going to jail and to protect his son from the stigma. He'd lost his other daughter a few years before so may be more protective of his family. It's possible that the son may have done it and that the parents were covering up for him. But you would still expect to see the parents more visibly upset. They just seemed rather cold and composed all the time.
I really do think that it's the family colluding here.