Poor, poor Leah; truly the stuff of nightmares. And her poor family, we were all desperately hoping that they would at least get some answers after all this time. Barring accessing things like NM's search history posthumously for example I doubt we will ever know for sure the true sequence of events, unless he confessed to someone before he died who has kept quiet about it up until now.
His occupation certainly seems to answer a lot of our questions, as does the fact he is no longer alive – perhaps his intention had been to return and move at least her possessions once the search had 'cooled down', but he took his own life before that point was reached.
In terms of how he managed to get her into the house, I can well imagine that if she'd walked past him regularly on her way to and from work and they'd perhaps said hello/briefly chatted before, that her guard would have been down if he'd asked her for her help with something inside the house – just off the top of my head, someone saying to me 'there's a wounded bird/animal in the garden and I need someone's help to rescue it' would quite possible override my own alarm bells if it wasn't a total stranger.