1. Nancy Minerva Scott was born in Pennsylvania abt. 1831. You'd have to find her and David's marriage certificate which might or might not provide more information.
2. I searched in Ancestry and FamilySearch and couldn't find the 1870 Census. Name was probably mangled in the Census and indexed under something else.
3. I believe David and Nancy did have a son Gibson. I didn't find that in any records but did find him listed in the Adams Family Tree, trees.ancestry.com/tree/10462834/person/297738924 . They just listed his name and nothing else.
3a. I think William McKinley is another mystery. He was listed in the 1860 White Oak, El Dorado Co., CA Census but then he disappears.
I found him on the Kastanek Family Tree, trees.ancestry.com/tree/7877450/person/524346167 . He was born 10 Sept. 1852 in Niles, Trumbull Co., OH. That's all that's on this tree.
I THINK I found him on the 1900 Cincinnati Ward 18, Hamilton Co., Ohio Census. William McKinley, b. OH, Feb. 1853, single.
I didn't find any death info for him.
There is another William Henry McKinley on the Vance/Glass Family Tree, trees.ancestry.com/tree/9546999/person/-19239734 .
William Henry McKinley, b. 5 Jan 1853 Circleville, Fairfield Co., OH. He lived in Illinois, married and had many children. He died in 1926 Chestnut, Logan Co., IL.
David Allison McKinley is on the Montana Porter's Family Tree, trees.ancestry.com/tree/6431934/person/-648833589 . He was born 1830 Fairfield, Columbiana, OH and died 18 Sept. 1892 in San Francisco, San Francisco Co., CA.They list his wife as Nancy Minerva Scott b. 1832.
You can verify his death info by requesting a copy of his obituary (if one was published). Go to freepages.genealogy.rootsweb.ancestry.com/~obitl/volca.htmland scroll down to San Francisco Co.
I believe that Ida Helen Morse died either in San Francisco county after the 1900 Census and before the 1920 Census. She might have died in Los Angeles, Los Angeles Co., CA before the 1920 Census.
You can take a look at the San Francisco Genealogy, sfgenealogy.com/sf/sfdata.htm web page.
The San Francisco Genealogy Society, californiaancestors.org/ might be able to find her death info for a fee.
You can contact Office of County Clerk, sfgov2.org/index.aspx?page=64 for a copy of her death certificate.
If you don't find it in San Francisco County, then I'd try Los Angeles Co. There is a vital records volunteer at Random Acts Genealogical Kindness, raogk.org. Choose California Choose J thru L Scroll down to Los Angeles Co. and choose Kathryn or Betty.
If they don't find her death date, the Southern California Genealogical Society and Family Research Library, scgsgenealogy.com/catalog/index.php?cPath=34 will do the research for a fee.
In 1920 George and Marjorie were living in Los Angeles Assembly District 73, Los Angeles, CA. George Morse, b. CA, 49, W, driver creamery. Margaret Morse, mother, b. NY, 74, W. Margery Morse, dau, b. CA, 16. Sadie Brown, roomer.
I'd look for George's death info and Marjorie's marriage info at RAOGK as mentioned above. Successful Searching!