I would very carefully snip the threads holding her to the muslin, then hand stitch her to a new piece.. I had a sampler that got stored in a box that a mouse got into and he left a "calling card" on it that left a couple of stains that look similar to that.. I also tried everything and never could get it out.
I hope the "home remedies" work better for you. Think I'll try the vinegar and baking soda myself.. and maybe some toothpaste with whiteners. it can't hurt mine..