On the question of planned vs impromptu, I'm seeing lots of signs of pre-planning. Not to say that there wasn't a level of improvisation during the actual crime. As a (fairly) inexperienced criminal his planning was just not adequate enough. He made rookie mistakes. But that doesn't preclude that he made a plan to kidnap, rape and murder Alesha specifically.
The timing at the end of the school year and hosting a party at home, gave him lots of people that saw him. I wouldn't take his word about how much he said he drank or that he was so stoned. Much of the testimony of witnesses are them repeating what he told them about drinking or feeling depressed. All that self-reporting is unreliable. Useful for him to have a witness pass on that information as factual. It was him setting up an alibi IMO.
The boasting and attention seeking behavior of posting a video to claim that "he did it" to his friends can't be written off as juvenile behavior. He was proud of pulling it off, but couldn't take direct credit. But he drew attention to himself in the community. A 16 year old excluding his mother from the initial police interview would be unusual. He was crafting his story to fit whatever the police were throwing at him without having his mother contradict him on the facts.
He picked a relatively easy to control sleeping 6 year old victim. On the flip side, a sick child murder brought a lot of attention and police and community effort to identifying the assailant. He probably knew the general patterns of the family and how easy it was to gain entrance. He probably did a trial run.
Didn't he know about the CCTV at his house? I think his mother might have wanted in general to keep an eye on him, so kept it on even when the relative with dementia was no longer living there. She's caught him in lies before, but had some niggling doubts about him and his behavior. Picking a high profile victim and his cockiness seem to have quickly undermined his plans. Kidnapping is not a haphazard spur of the moment action.
