Steve,
By sheer chance, I research both the Casteel surname (not in Campbell Co.) and many of the early families of Campbell Co., TN ... so I have an interest in your problem.
I agree that Elizabeth Casteel on the 1920 Campbell Co., TN, census is Elizabeth HILLIARD (or some spelling variation), 2nd wife of James Casteel of Laurel and Rockcastle Cos., KY.
The 16 Aug 1895 marriage in Rockcastle Co., KY, of James T. Ross to Almira Casteel, plus their 1900 census appearance in the same county before moving to Campbell Co., TN, is good proof that your Almira/Myra is the same woman who appears in 1880 as daughter Almirah Casteel to James Casteel and 2nd wife Elizabeth Hilliard.
The only glitch in what you have spelled out is your 1910 identification of Elizabeth Casteel. I don't know where your Elizabeth HILLIARD Casteel lived in 1910, but the woman you found in Monroe Co., KY, should be Elizabeth STRONG Casteel, daughter of Joseph Strong/Mourning Denton and widow of William Casteel.
Contact me through my email address,and I'll send you some 'stuff.'
Leslie McConachie
lesliemac100@sbcglobal.net