I also have that tidbit in regard to the 1860 census and a 43 year old John Kent in Adair County.
The only info I can give you about the CSA John Kent is in regard to his Compiled Service Record. He was a member of Company K, 1st Missouri NE Cavalry (under Porter). He enlisted on August 3, 1862 (3 days before the battle) by Colonel J. W. Duncan in Schuyler County, MO.
I have been unable to locate any official list in regard to those executed at Kirksville. The only information comes from County History books written years later and they are notorious for being inaccurate when it comes to giving correct names. (It is known that they were wrong in regard to some of the names of the Union killed at the battle)
The fact that Mrs. Otis Miller expressly mentions John Kent by name lends credence to the fact that he was one of those executed. I do not know if there are any other Kent families in NE Missouri. I suspect that the CSA John Kent is a different one than the 43 year old Adair County John Kent that joined the 32 MO Infantry. (It is possible that the CSA John Kent was captured at Kirksville and then given parole on the condition that he join a Union regiment).
I have recently moved and do not live close to the microfilm Compiled Service Records at Columbia, MO. A look at the USA John Kentís record might shed some light Ė or it might not. Also, the pension record may or may not shed light.
Someone with access to a searchable census database may be able to locate any other John Kentís in NE MO in 1860. If you find out anything, please let me know - until then, you have a conundrum.