I'm not sure what I'm seeing, beyond a not so good plaster job. There's a lot of reflection and also some image curvature due to surface ripples.

+ How exaggerated is the coloration in the 'enhanced' photos?

+ Is the plaster really as rough and uneven as it appears in the #2 enhanced detail and in the #3 enhanced views?

+ How did you test for metal stains?

+ Have you tried dropping a bag of calcium hypochlorite on one of the stained areas? If it's any sort of organic stain, that will tend to lighten it.

+ It *looks* like dirt stains in somewhat sloppy plaster repair, to me. Is that possible? Mineral based stains (like embedded clay or chert soil) is not removable.