I just wanted to comment on your idea of a flannel with the cotton batting ...
I had a quilt that I had sewn strips on to a lightweight cotton foundation, then the backing was going to be flannel, so I put flannel on the inside ... my little machine handled it just fine. With an industrial machine, it will be like sewing through butter. And, I am going to keep in mind the doubled batting as well as a flannel WITH batting.
Do you think that the amount of quilting can also make a difference as to how warm a quilt is?