Thank you, LanitaSS1. I had already checked the Missouri Marriage Records at Ancestry and wondered the same thing - that perhaps not all of Grundy had been reported. I believe you've found proof of what I already suspected. A marriage in Grundy County, MO never made sense to me.
The 1899 record would not be them. My g-grandmother was born in 1891 and her first marriage was in 1909. My grandmother was born 1912 as Hazel MONROE. She is found as Hazel ADAMS in the 1920 Census (Fulton County, IL), Hazel ENSIGN in the 1925 Iowa State Census (Davenport, Scott Co.), and is missing from the 1930 (They were living in a back house that wasn't recorded in the census but were still in Davenport, IA). Since she was married in 1931, I know I will never find a census record with her given name.
The 1925 Iowa State Census listed Jacob ENSIGN (JL Ensign) as her father and Ella MASEKER as her mother. The two children of Jacob's marriage to Hazel ESSLER are properly identified.
In this particular census, it asks the marriage location of each person's parents. Under "Hazel Ensign" (should be Hazel Monroe), it states that her parents (with her father identified as Ensign, not Monroe) were married in Trenton, MO. Hazel's parents, Charles L. Monroe and Ella Maseker were married in Kewanee, Henry County, IL. HOWEVER, Hazel Essler's parents WERE married in Trenton, MO. I wonder if this is where the confusion begins - two Hazel's - one the mother of the other two children in the home and one the daughter of divorced parents, a father whose surname is erased from this and every other census record.
Thank you so much for your help!