Only once did I not put batting in a quilt because the backing was flannel and I thought that would be enough. I have regretted it ever since. There is no definition to the quilting and it just looks flat even though I love the fabrics.

I have had a shower curtain for years with no batting between front and back. I just used cotton on the back and quilted around whatever on the front. Works fine. I use a shower liner between curtain and shower.

Yes, I would suggest using a shower liner to keep the curtain from actually getting wet.
I quilted a table topper without using batting of any kind. I found it much harder to quilt.

I have made several shower curtains with heavier cotton, some lined and some not lined. I have not made a pieced shower curtain, however. What I can tell you is that I like the unlined shower curtains best because they laid better and dried quicker. If I was making a pieced shower curtain (and this does sound pretty), I would line it with a light weight cotton and SID. How are you doing the top? I would recommend a separate piece for buttonholes or grommets and that piece I would interface or use extra lining to stiffen. Can't wait to see it.

Just a thought. Why not use rip stop nylon as a backing. It is light weight and waterproof.

I made mine years ago, without any kind of backing, because I thought it would dry better and faster if it were thinner. I just zig zag finished all seams, and made sure to sew over all seams that were not back tacked. I put a heavier clear vinyl curtain under it, and have been washing and using it for about fourteen years now and it still looks fresh. I guess it might be time to make another one, but I would still make it navy blue and sea foam green to match the bathroom though.