No hacksaw marks on window = no entry from that window 
Says you straight of the top of your head. Who said he used a hacksaw the first time? He mentioned gaining access previously with the use of a pen knife. That's a BIG BROOM you use for your 'sweeping statements'.
No scratches doesn't mean no entry! What you also fail to consider is that the window may have been left open either by the Bambers, or by Jeremy before leaving WHF. He also doesn't have to have used this particular window. Any window may have been left open - I doubt the police checked the windows that thoroughly given that they thought it was 4 murders and a suicide, especially when they had a witness who gave them half the story before entry. If a window wasn't latched properly, I doubt they even noticed and they opened windows themselves later in the morning so entry could have been accessed through any window with no evidence to show for it.