I had always heard that if you use lemon juice on the stain it would lift. Since you have done so much to the pants already, what would a little lemon juice matter? Give it a try
If it seems practically impossible to remove the line, you might consider sewing some kind of decorative strip over the line, such as lace (if dressy) or bias tape or some other type of trim.
If you know someone that makes homemade soap, buy a bar. It works wonders for removing stains. There is a Castile soap that is sold in stores but I'm not sure of the brand name.
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