I've used lemon juice and sunshine for years to get mildew and "unknown" stains from the boys clothes, etc. I never used salt with it, just pour pure cheap store brand lemon juice on the stain, hang or lay in the sun and leave it hang until you see no stains. Then wash as normal. I've never had it weaken or bleach out the fabrics, but if you're worried about that, test it first on a small patch.
Hope it can be saved! Good luck!