Who would have been passing by the house at WHF at that time? They would literally have been on WHF property. He left by the window so that it would LOOK like an inside job.
The guy who was walking his dog heard a shot,which must have been well after Neville had been out with his dogs,as he'd normally leave the farmhouse between 10 and 10.30 pm.
I wouldn't " buy " leaving by the window if I thought he was guilty.There'd be neither the point,nor the sense in doing that,and besides,you've just said yourself that nobody would have been around,so why not leave by the front door to save squeezing through a window which would trap any fibres and leave all kinds of evidence ? As it happened,there was no proof that anyone got through any window.