The most likely person to know about a lockable from outside window at WHF is Bamber -
He used to live there. As a boy and a man.
He still worked there.
He said to the police he knew how to enter and exit WHF through windows.
If he was entering and exiting an empty WHF prior to the massacre, he would have to leave all windows shut. To deter burglars and not leave a trace he had been inside.
If he didn't know in advance, he could just check all windows to see if any could be banged shut from outside. There were enough windows there. If there were not any, he could spend a few minutes adjusting a window a few days before the massacre so it was possible.