There's no question about a prior water issue in that area, and no one will ever know what kind of products the homeowner used there.
The pictures, if the color balance is correct, look like an alkaline burn. This is exactly what happens when a consumer hears "soda water" to rinse a urine spot and uses baking soda instead. Or, a tech uses his restaurant grade nuclear oxidizers on wool.
A rinse and treatment with a very low pH (like a rust remover) might help.
Faster and cheaper than dinking around with a hundred spotting attempts would be an insert.