Take your completed top and piece of fleece, now lay on the couch with both pieces. It may be quite warm without any batting. Just a thought. Yes, you can use flannel for batting, wash a couple of times because of shrinkage, flannel is notorious. Make a small sandwich, just pinning it together will be fine, now try different binding techniques until you find one you like. And yes, some use the backing by just wrapping to the front. When I took a binding class, the made a good sized sandwich and tried different techniques on each side. Goggle videos for different binding techniques.