You wrote:
“Apart from that, I recall reading that Charleston had dredged the channel again on their own dime around 1860.”
The real story:
“The Rivers and Harbors Act of 1852 authorized the federal government to dredge the channels of the harbor to a depth of 17 feet. This deepening work was interrupted by the Civil War and was not completed until after the war’s end.[3]”
] U.S. Army Corps of Engineers, Charleston Harbor Post 45: Final Integrated Feasibility Report and Environmental Impact Statement, Page 1-6. via Wikipedia
The existence of this effort does not preclude the possibility of a local effort to do the same. My recollection is from reading accounts of the boom conditions occurring in Charleston after secession was declared. Warehouses were being built, Hotels were full up with no vacancies, New docks were being constructed, new businesses of every stripe were being opened and there was a large migration of Northern craftsmen showing up in Charleston to provide their services, and so forth.
This is just like the Navigation act of 1817. (Which you didn't believe existed or that it had the effects I described.) Charleston was going through a massive boom build out during the secession period, and it was all in anticipation of massively increased trade as a consequence of getting out from under USA tariffs and other trade restrictions such as the Navigation act of 1817.
I haven't read this information in years, but as I read it originally, i'm certain it can be found with sufficient looking. If I run across it again in a timely fashion, i'll send you a link, but i'm not going to put myself out to find another thing you don't want to believe until you can deny it no longer.