I hope the dye catchers work but. . . years ago, I had an art teacher who said, "Barbara, if you can't draw a straight line make it look like it's SUPPOSED to be crooked." I don't use it often but I did take it to heart and it's bailed me out of a few nasty situations. Sometimes in very interesting ways.
If you were to applique a bias strip of fabric, a ribbon, embroider a line or couch a wide decorative yarn down the center dark dye line. And if you repeat it somewhere else in the quilt, it just might look like it was supposed to be there. Look at that one picture, doesn't the dark line look embroidered? Hey, it better that tossing the whole quilt.
Good luck, it's a lovely quilt and well worth saving.