V off the wall theories here - bear with!
If Leah was in the habit of varying her route to work for safety reasons, I don't think it would be in keeping for her to follow someone into their house (even if she had said good morning to them before) or to get into a car with a stranger. She was young, but she was trained in self defence, which makes me think that her parents had made personal safety a priority and it was something that she took seriously.
I am concerned about her sudden secrecy with her phone - switching off location on the previous evening and (if the later sightings are to be believed) possibly using a "burner" phone.
Earlier reports mention that she used snapchat, shortly before her phone was turned off on the day she was abducted. Snapchat is quite a secure way to communicate. Could she have messaged someone to say "there in 2 minutes" and then turned the phone off? Do the police know whom she had contacted that morning? I think these answers are really important.
NM is reported to have successfully evaded arrest by changing his mobile phones to avoid being tracked and located. Could he have "taught" Leah to start using this slightly crude tradecraft to cover her tracks?
Could NM have started a friendshio with Leah - by chatting to her over the previous few months and then spun her a story about how he was an undercover cop/spy? Hanslope Park is just around the corner (HMGCC). There must be more than one Walter Mitty around, trying that sort of thing on to get attention/girls. It would also explain his sudden absences to avoid the Police - he could pretend to Leah that he was on important business...
Maybe he convinced Leah that she was vital in helping him in some secret operation. If Leah had a vivid imagination and enjoyed fantasy fiction, this could explain why she was being secretive around her parents and not wantonly taking liberties with her safety - if she thought she was involved in something "top secret".
I know, I know, it's a long shot. But the fact is that he was canny with his phones and was clever at changing his loation and possibly also his identity to dodge the various police forces after him.
It is much easier to dismiss NM as crude and perhaps not that bright, but I am not sure that was actually the case. He also managed to convince the owners of the property to trust him with their only key. It's quite possible that he could turn on the charm, when required.
For me, there can only be two scenarios where NM is the perpetrator that work - the first, that her abduction was an opportunistic attack that nobody witnessed, coming straight on the back of some secretive and unusual behaviour from her, that had to involve another party. So just really unfortunate timing.
Or, that it was a premediated murder with a complex backstory in which he gained Leah's trust.
Leah knew NM. He had "trained" her in understanding phone tracking. He had organised a hotel meeting where she was dropped at a fake location before moving to the actual rendezvous point, helping her to believe his story. He then convinced her to meet him the day before briefly when she turned off her phone - some kind of quick briefing? The the following day she snapchats, turns the phone fully off and then goes to the house, then he attacks her.
And then there is the scenario where he is just an accessory to Leah's murder. Could he have handed the keys over to some other unpleasant character in return for money? Then, when he came back - discovered what had happened and killed himself, because no one would believe that with his criminal record, he could be innocent of this crime?
Just throwing some (admittedly fairly far-fetched) ideas around...
