I, too, did a lot of crewel, as did my mother. It's not as delicate as one might think. In colonial times it was used to decorate footstools, chair pads, clothing, bed linens and all sorts of 'functional' items. I have several pieces on footstools, pillows, and chair pads that are holding up just fine. Many were done in the 70's. Today I use crewel yarns for embroidery on crazy quilts and art quilts all the time.
Warm and Natural batting can be quilted as far apart as 8 inches and is perfect for wall pieces. And you can always fuse the top to the batting if you want to quilt further apart than that. Personally, I wouldn't use your pieces for anything that will get frequent machine washing, but such pieces can be gently hand washed with no real problems at all.
I like the idea of the embroidery as a central focus with borders of different widths around it. You could also add an open embroidery stitch, like a feather stitch, on the seam lines of the borders, and it would unite the outer portions to the center.