I agree with several others to use the narrow grey all the way around the entire quilt. I like the idea of an even piano key border using all of the fabrics used in the quilt in a random order. I would use the red for the back of the keyboard area.
I would make the piano border at least 1/2 tp 3/4 the width of what your squares are if not the same length.
I would measure the bed checking to see how high the bed sits off the floor. You can make the quilt larger if you want it to hang down the sides of your bed.
You have a wonderful quilt and a precious memory keepsake for your family.
Thanks so much for sharing with us. Keep us updated on the progress of your quilt.
Pam M