Wow, we are doing the same thing this very day. Several cushions had light mold/mildew stains (though didnt really smell). We were afraid if we took the covers off we would have trouble getting them back on, as well as afraid the fabric would disintegrate in the washer. My wife wanted to clean them on the cushion, but I wanted them cleaner than that even if they were going to end up in the trash in the process.
I monkeyed around three days trying to loosen up the zippers, and gave up. The sliders were so corroded I finally cut them off. If we cant get sliders put back on I'll just sew the zipper shut.
But they didnt disintegrate, and they came out looking really good and smelling fresh. Were going to wash them one more time with some lysol (the brown bottle), and start putting them back together. The check list is nearing the end.