Mary and DWC had many slaves. Slaves frequently took owners' surnames as they were freed. It could have been one of their former slaves. Mary Gwinn Bonham moved with her daughter and son-in-law to California after DWC died.
I just returned from Vicksburg where I did some research. The area of the former estate has been inundated by the Mississippi River. It's a possibility that it is now situated in Louisiana.