U. S. Census for 1880 has a William F. Maberry, born in Roane Co., TN, 1851. No Willis Maberry in Tennessee Census records for 1880 (four years before the killing).
Two men with the name Thomas Galbreath are listed in the 1880 Tennessee census, birth dates 1845 and 1859. Neither is listed as resident in Roane County at the time.

The Court records are archived in the old Kingston, Roane County Court House, an attractive pre-Civil War structure badly in need of repair (a heritage group is trying to raise the 1/2 to 1 million dollars needed).
It would help if the dates and exact summary details of the trial and sentence were copied. Also, where did the 'first rendition' come from?

Of course this has nothing to do with whether the Blue Sky Boys borrowed an older tune, or if that tune was the basis for "Precious Jewel" as recorded by Acuff.

The reference to a name change in "The Killer Poet" refers to the village of Oakdale-Elverton, not the County. No 'Mayberrys' are in the Census for Roane County in 1880; the spelling 'Maberry' seems to be the correct one.