Here is the 1870 Census record you cited, in case you want to add it to his prolfile:
"United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M46F-897 : 12 April 2016), Robert Minium in household of Peter Herington, Missouri, United States; citing p. 25, family 178, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 552,286.
And here is their marriage record. If the marriage is in an index, there's a good chance a certificate is available, and said certificate may or may not have info on parents, witnesses, etc, that could prove valuable.
"Missouri Marriages, 1750-1920," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:V28B-XDW : 11 February 2018), Robert Minions and Martha Elender Pitcock, 18 Mar 1880; citing Ozark,Missouri; FHL microfilm 929,570.
I'll keep hunting.
Both FindaGrave memorials are 'Non-Cemetery Burials.' They have specific info, but no indication of the sources. A death certificate would show his parents, if the person reporting knew who they were. OK.gov website is having some issues right now.
OK record search is a no-go for now. At Ancestry, there are seven 'private member photos' related to him. Have you tried contacting that Ancestry user?