No idea who the photographer is. Newcomb? Date - I'm guessing late 1880s-1891, when the fort was abandoned. Fort Bennett was on the Cheyenne River Agency
The man near the left is Louis DeWitt, who had scouted for the army in the mid-late 1870s.
This last image is a Newcomb and depicts the slaughterhouse. No date, unfortunately. Newcomb seems to have been there in 1884, but whether he was around to take the later images, I'm not sure. Bearing in mind the fort's significance during the Ghost Dance, I wonder if the first four images were taken around that time, specifically that winter, judging by the weather. Speaking of which, the Ghost Dance, that is, doesn't the man second from the right of the DeWitt photo bear a resemblance to Kicking Bear...?