I have almost no certain knowledge about Nancy Walker. To estimate the date of her death, we must look at the marriage of Susan McDonald and her first husband. Nancy appears to have given birth to daughter Rebecca July 1888, and it is possible that she died in childbirth or shortly after. It is also possible that she lived to give birth to Cora February 1895. I can be sure only that she was dead by 1900.
Susan was born about 1854 and married James B. McDonald March 1877. He was dead before 1900 at which time Susan was married to James Calvin Smith. Cornelia McDonald, born in January 1887, was the youngest surviving child, which means that J. B. McDonald must have lived until some time in 1886. The parentage of Cora Smith, born February 1895, is key to the deaths of both Nancy Walker and J. B. McDonald. On the 1910 census, Susan stated that she was the mother of seven children, six of who were living. The living included Lula, Wellington, Lillie, and Cornelia, and I have found statements not supported by evidence that there was a son William. That leaves at least one child, which means that Cora could be the child, but I must leave the issue unresolved.
1880 Oregon County, Missouri, page 363A stamped
J. B. McDonald, 23 MO SC TN
Susan M. E. McDonald, 25 TN TN TN
Lula McDonald Dau, 10mo
1920 Ripley County, Missouri, sheet 7 211A stamped
Wellington McDonald, 38, AR MO TN
Belle, 30, MO TN TN
Reva L., 7, MO
Dorthy, 4y 4mo
Doyle B., 2y 7mo
Susan E. Smith, mother, 69 wid
James Calvin Smith to Nancy Catharine Walker, 26 Jul 1874, Ripley County Court Records of Marriages, Doniphan, Missouri, Volume D, 1866-1881, page 88
Susan's previous marriage: J. B. McDonal to S. M. E. Cantrell, 29 Mar 1877, Oregon Co., MO; Oregon County, Missouri, Court Record of Marriages, Volume 2, 1877-1881, page 10
His name was almost certainly James. The family listed immediately before J. B. and Susan in 1880 was the family that included James of the right age in 1870.
(Gravestone, FindAGrave: J C Smith, born: June 8, 1855, died: Jan. 10, 1911)
1880 Oregon County, Missouri, p. 7 362C stamped
James C. SMITH, 25 TN TN NC
Nancy C., 26, MO TN TN
Irvin J. SMITH, Brother, 17 MO TN NC
1900 Oregon County, Missouri, p. 64B
J. C. Smith, June 1855, TN/TN/TN
Susan Smith, July 1854, TN/TN/TN
Cornelia Smith Apr 1887 MO/TN/TN
Rebecca Smith Jul 1888 MO/TN/TN
Cora Smith Feb 1895 MO/TN/TN
Lillie McDonald Oct 1884 MO/TN/TN step-dau
Cornelia McDonald Jan 1887 MO/TN/TN step-dau
Isaac Smith, b. Dec 1880, MO/TN/TN relative
1910 Ripley County, Missouri, sheet 12B 112 stamped
James C. Smith, 54, TN TN NC mar 34y (supposed to be years of present marriage)
Susan E., 55, TN TN TN 37y 7/6 (number of births/living children)
Beckey M., 20, MO TN TN dau
Cora B., 14, MO TN TN dau