[If I do the SID- do I go all around the outside and inside the 5 inch block or in each of the blocks within the 5 inch block? Do I do this with a walking foot or free motion foot? I like the idea of the serpentine stitch -- where would you do that?]
Use a walking foot for a Stitch in the Ditch or even the Serpentine . I would try to get some "balance" in the quilting in other words even amounts in the distance between quilting through out the quilt. this will help with the "looseness" you describe. I would go around that inner signature square with either a Free Motion scroll type or Stitch in the Ditch. The Free Motion might be more forgiving , and be consistant with the other quilting.