JanRN ... Oh dear ... and I'm hoping this is not the decadron.
I don't know a lot about fusible battings and only used it once in a purse.
I do know that when I was purchasing it, there were different stiffness levels. The stiffest seemed all board like and would let it stand up on end ... which would be nice if you wanted a stand up tote for groceries! Not so otherwise.
Do you have some of the batting left?
My suggestion would be to make a mug rug or placemat, ideally with the same fabrics as your wall hanging. Try to copy it as much as possible ... fabric pieces about the same size as your wall hanging, and quilting about the same distance apart as on the wall hanging. Then wash it and see what happens.
I'm pretty sure it was
Tartan that posted recently that she uses fusibles a lot on her bed quilts. So she may have some insights for you.