I think an open verdict is a better conclusion. There is no evidence she chose to leave the house unaided through the window (no fingerprints) and while it wasn’t impossible that she did so, for example, by kicking it open and then lowering herself on her tummy, it was surprising that she’d choose to do this.
I’ve always considered that there could have been some kind of involvement from other people- not necessarily with malevolent intentions. The whispering in the house remembered by the mother when she had been half-asleep.
All possibilities seem unlikely, but one or more of them took place. The open verdict acknowledges the range of the potential “unknown” events, given the unique person Nora was and the unlikeliness that she’d just wander off and scale fences etc. I hope her parents feel they have all been more acknowledged as a family, even if the truth never comes to light.