I don't know if anyone is still interested after more than a year, but here's what I have anyway. There is obviously no way to prove it, but appearances are that Cora's biological father was William Myers, the husband of Nancy's older sister Catherine. Circumstantial to be sure, but names, dates and places all fit the assumption. Our family history (I'm a distant collateral relation to the Vanamans) always refered to Cora as "Cora Myers-Nixon" long before hyphens were in vogue. I was, however, totally unaware of John Ensley. That's probably why I haven't been able to find anything about Nancy between 1870 and 1880.