Adding a layer of batting I think would help to stablize it and if your end goal is to hang this beauty the batting will help it hang straighter. It will also absorb many of the lumps and bumps and spongyness of the seam allowance. A thin batting would work best, i think but something with a bit of body to it. It will of course add to the weight.
I would recommend avoiding ditch stitching the pixels. I think contouring the face would have more impact. Plus the quilter would most likely swirl those pixels anyway as the pixels represent shading and it would give the quilter an opportunity to swap out thread colors that would best blend with the shadows created. But remember a LAQ is working only about 18" away from the work so much of the shading would be lost on him or her in the quilting process. It would probably require marking areas where a change in thread color would have the most impact as well as marking areas that are the nose, cheeks etc. The portrait won't be visible to the quilter once it is on the rack and only 24" or so are exposed at a time.
I was wondering what the letters were in the background. Thanks for saying. Having a quilter highlight that can be done but it may not warrant the cost. the letters are on 1/2" squares, that would be some serious detail work. But the area would have to be quilted in some way.