I've been quilting the middle of my 100" wide quilt for a few weeks, using a high table or bed to hold the quilt and sitting in a chair next to it. The easiest way for me to manipulate all of the fabric is to reach up under the quilt with my non-dominant hand and arrange the fabric to the left of the stitching area like a mitten. The fabric lays smoothly over four fingers and palm of my left hand and the thumb is encased in the folded quilt. The four fingers can manipulate the fabric as needed.
I rock the needle out of habit, but after that I use my thumb on the other hand to fold the fabric down and shorten the stitch length when the needle comes back up. The index and third finger are spread out a little so that the needle comes down between without pricking. Controlling the smooth tension of the layers is achieved with those three fingers. It works for me.