As you have already washed it. the lint issue will be less. I (personally) would lightly starch it - I think that it will be easier to work on. As for the stitch size and seam width, I always use a 1/4" seam for flannel and a slightly shorter length stitch. I think that I have only seen the 1/2" seam recommended for the rag quilts. I would not worry about zigzagging the edges. As these seams will be inside there will be minimal friction on them when it is quilted.
My only other thought is perhaps you might want to consider pressing the seams open. If the snowballs will be sewn to one another, and you press all the seams in the same direction you may end up with a good size lump where the seams come together due to the thickness of the flannel.