If you sell an essay to a magazine that purchased all rights (and you were paid) and that magazine folds before your piece is printed, what are your rights in terms of placing it elsewhere?

The publisher retains the rights. They will likely sell those pieces either to another pub, or as part of the package if they sell the magazine. Even if they never sell the articles, they still own them because they did pay for them and the agreement was they were purchasing all rights.