If it won’t wash out of your border fabric, I wonder what it does inside the quilt? At this point with all the things you have tried, I would replace the piece of fabric or cover it with an appliqué. Sometimes with a pieced quilt, I like to extend the pattern with a partial quilt block into the border. It can give a nice look and can help disguise having to piece a border. The opposite of Happy Dance? Slowly banging your head on the sewing table.