Since you're stab stitching the quit, I wonder if it would be easier to do it in a lap-size hoop so you could turn it over to see what you're doing on the bottom. However, you'll run into trouble with those pins using a small hoop; thread-basting is a better option for hoop quilting. I've never worked with so much batting, but I wonder if you couldn't do some big stitch quilting using a rocking stitch? The only options I see for your present set up are to rig up a mirror so you can see the quilt bottom, or one of you get under the quilt.