An update on research of interest.

[SSlater]

When Harney was received at Elmira Prison, 2 July, he had certain regulations to Follow " sign this - sign that" etc. And he declared that he wanted to sign "The Oath". and he stated that " he wanted to return to his home in New Orleans." Then on July 7, he signed the list of those who had signed the Oath, and declared his place of residence was LEESBURG, Virginia.
This is more in line with the information I found where - a month later he joined a U.S. Army unit in Maryland (A few miles up river from Washington. That's getting close to Leesburg.)
I suspect he had more active duty time in the U. S. Army before 1850 and later than 1900 (in the Spanish-American war.) But proving that this is the same "Thomas F. Harney", I can't confirm.
I'm working on it.