One more attempt at a yellowed stain...

and I don't notice a big difference.  At this point I think I've done all that I can and am willing to try to remove the stain.  I don't want to destroy the quilt by experimenting beyond the soaking and using a Shout Color Catcher.

The first picture is from after I finished the applique and before the binding, the second picture is after the second attempt at soaking.  I tried to keep the lighting the same, and no photo alterations other than cropping.  It might be a little brighter, but for the most part I don't notice a difference in stain removal.