I think your problem would be solved if you apply several layers of spray starch to the top. You can lay the quilt out on an oversized flat sheet, spray from the sides towards the center (to minimize overspray), turn on a fan to speed drying, then apply another layer.
While the wool batting may be contributing to the problem, my guess is that the primary problem is caused by the fabric stretching. Starch will stabilize the fabric so it's not so easy to stretch. Are you able to reduce the presser foot pressure on your machine? I am thinking that the presser foot is exerting too much pressure on the top fabric because the quilt sandwich is thicker than normal.