I like the fabrics you used and the way you put them together.
At this point, I think I would just leave everything unwashed. Put the flannel backing on as planned.
Tie it (at least every four inches, if not closer) and - when necessary - wash it in cold water and machine dry it on "low" or hang it on a line to dry.
It probably will get a little "weird" but it it's a "quilt to be used" - I would just finish it and let it be loved.
As far as batting goes - depends on what you plan to do with it. Sometimes, just a little extra warmth is all one wants.