Ditto about the poly batting, it will not stop the shrinking, it will make it look puffy. I'd go with a needled cotton on something like that, so the eye sees the beauty of your quilt.
As for quilting. You could stitch in the ditch around all of the little diamonds of the very most center, and then as you have the border going around that - stitch in the ditch on both sides of the border. Then measure the distance there, using a quilting foot offset out that distance with your diamonds getting bigger and bigger.
Once you have sewn all that you can stitch in the ditch on all of the the other "diamond borders" Which will tack you down and in, preventing any shifting.
It sounds like a lot of quilting, but it goes quite quickly.