Thanks Carol. Well I couldn't find Sarah's death (Jackson's wife, mother to William) even with the added information. A mystery.
Some of what's below you have but it might hold clues to anyone trying to help. Some may be new.
As for the name of the mortuary, I would think just about any issue of the local newspaper from that time period would have notice of a death using the services of the Richards funeral home. There might be a city directory also, not digitized and available online. In all the records I found for William in Idaho, he is a farmer.
World War I Draft Registration Cards 1917-1918 (hard to read the date on it) William F Richards Ada, Idaho Occupation FARMING Birth 28 Oct 1880
WWI Civilian Draft Registration Name: Richards, William F. Birth Date: 28 Oct 1880 Birth Place: lives at RFD Melba ID City/County: Ada State: ID Ethnicity: W
1920 Pleasant Valley, Canyon, Idaho William F Richards38 b abt 1882 in Pennsylvania both parents born PA FARMER Bernice L Richards32 b abt 1888 in Michigan both parents born Canada Mildred E Richards7 b abt 1913 in Idaho Bernice Richards6 b abt 1914 in Idaho William J Richards 4[4 1/12] b abt 1916 in Idaho Mabel Richards 0[5/12] b abt 1920 in Idaho
1930 Lawrence, Whatcom, Washington William F Richards 48 Bernice Richards 43 Mildred Richards 17 Bernice Richards 16 Jack Richards 14 son [must = William] Mable Richards 10
1940 Lawrence, Whatcom, Washington Will F Richards 59 presumed residence in 1935 - same house; street "gravel road north of Lawrence" Bernice Richards 54 W Jack Richards 24 Robert F Richards6 son
World War II draft registration Name:Willam Filson Richards Birth Date:28 Oct 1880 Birth Place:Monongahela, Pennsylvania Residence:Rt. #2, Everson, Whatcom, Washington Telephone: Everson 805 Race:White Employer:Self Name and address of person who will always know your address: Bernice L. Richards same address Signed Will F. Richards
William died 1955 buried Ferndale, Whatcom County, Washington Bernice died 1958 buried Ferndale also I looked in that cemetery for Sarah Filson Richards but did not find her.