You said: “...virtually all cotton exports left from Southern ones (ports).”
Not correct. According to your source, Wise, on page 229, Appendix 3, for the year 9/1860 to 8/1861, the city of New York was the forth largest exporter of cotton to overseas markets. It shipped 248,049 bales...more than Charleston, Galveston and the states of Virginia and North Carolina.
So, you are wrong.
During that period 91.2% of all cotton exported from the U.S. left from Southern ports, using the same statistics you're using. An overwhelming majority in anyone's book. Considering you can look at the fact that almost 95% of all tariffs were paid in Northern ports and somehow conclude that the vast majority of that was paid by Southern consumers, then I don't feel at all uncomfortable looking at 91.2% of all cotton leaving Southern ports and terming that 'virtually all'.