Mississippi River cruising on the American Duchess New ships, insightful port excursions update a nostalgic float   American Duchess, a 166-passenger modern paddle-wheeler of the American Queen Steamboat Co., is tied up at a port stop near Helena, Ark. (Photos by David G. Molyneaux and Fran Golden, TheTravelMavens.com) By David G. Molyneaux and Fran GoldenTheTravelMavens.com   Natchez, Miss.   To many Americans, the rural south of Mississippi and nearby states remains a mystery. Travelers seldom talk with people who live…