I was going to agree with the suggestion of wool, since one of your concerns is the weight of the quilt. Wool batting has the most loft, which means air, so it will make the quilt feel lighter than a cotton bat would. But the bearding is a concern if the backing is dark, so maybe Quilter's Dream Cotton would be the way to go, unless you would consider changing the backing to a lighter color. I think the quilt is beautiful, as is your quilted butterfly!