"You claim that she was a Union ship....??? She had mixed ownership, a home port in Charleston, and no mail contract when fired on by a Union warship."
As you well know, Nashville was not "fired on," in the same sense that Star of the West, another commercial vessel in Charleston Harbor was fired-on by South Carolina shore batteries on January 10. Instead, Harriet Lane fired a warning shot across Nashville's bow. As a result, Nashville raised a Union flag and then proceeded on to Charleston Harbor -- after firing on Fort Sumter stopped.
Your source, on page 445 clearly says the company's long-term mail contract expired in 1860. It does not indicate if Nashville was carrying US mail on some ad hoc basis in April 1861.
Again, my argument here is: for Harriet Lane's warning shot across Nashville's bow to amount to a "first naval shot" of the war, Nashville must at that time have been a Confederate ship -- which she soon after became, but clearly was not, during the period April 11 - 14, 1861.