I think my problem may actually be with the satin stitch. Just realized that years ago I did satin stitching using fine polyester embroidery thread and still disliked the satin stitch ridge and stiffness. I have hesitated to use a zigzag or blanket stitch on fusible, especially Misty Fuse, because I want this for bed quilts and am not sure I will like the results after washing. Also, silk thread seems so expensive to experiment with!
What I'm going to do, I think, is make a few samples with the Misty Fuse and zigzag using the polyester thread I have (which probably mimics silk thread pretty well), run them through the wash several times, and see what happens.
I did do some Googling on the web and found a few people who posted that the silk thread they used for hand applique has deteriorated. One said that she had used both cotton thread and silk in a well-used bed quilt and that years later she is having to repair all the silk, whereas the cotton thread is still solid. So, I am thinking that silk thread, at least when used for hand applique, might be better for wallhangings.