Natives of Matlock in Derbyshire don't speak with a Yorkshire accent unlike for example inspector Martin of the Derbyshire police.
Sherlock Holmes recovers a spent case from a round of ammunition, which would be the something found if a semi-auto pistol was used. However, Abe Slaney uses a single action revolver and the case would have remained in the wheel. There is no reason to believe that Slaney stopped outside the window to dispose of the empty case and reload that chamber.
At the conclusion of the story, during Dr. Watson's epilogue, modern vehicle traffic is passing on the road in the background behind the manor house in the upper left hand corner.
Screen shot of carriage leaving the estate at end shows a car and two trucks in the upper left.