I don't think it was her best presentation, primarily because as yet there's no crime scene. I suppose her conclusion would rule out the suicide theory
Yes, but I think she's come across the best reason for Andrew going to London he'd probably been planning it for a couple of days before hand.
Hence the reason why he was moody when he woke up late that morning. He'd left the house with the intention of returning.
Because of the way he'd left behind his school uniform PB thinks he may have intended to return home that same day, without anyone knowing he'd gone to London.
Maybe on his way to the station, he decided that being a Friday he would visit the grandparents with his new play station. And return home sometime that weekend
With no more sightings after reaching Kings Cross station, sadly he was picked up by a pedophile ring. Kings Cross being a favorite predatory hunting ground for pedos.