What assumptions did Holmes and Watson discuss about Hargraves being involved in Douglas' murder?

Holmes and Watson agree that Douglas did not commit suicide, and that someone from outside Birlstone House must be responsible for the murder. As far as Hargrave, Holmes rules him out.

“Well, Mr. Mac, it is very good and very clear so far as it goes. That is your end of the story. My end is that the crime was committed half an hour earlier than reported; that Mrs. Douglas and Barker are both in a conspiracy to conceal something; that they aided the murderer’s escape—or at least that they reached the room before he escaped—and that they fabricated evidence of his escape through the window, whereas in all probability they had themselves let him go by lowering the bridge. That’s my reading of the first half.”