I agree with Pam that I usually see them backed with the same fabric as the top. I probably would do 3/8" seams, pressed open to distribute the bulk. You could do 1/2" if you want, but it's probably not necessary.
I'm not sure what exactly you're making. It sounds like you are planning to add a batting, in which case, white would be fine except for the fact it is not easy to clean. In our church, people take turns bringing the laundry home with us & people exercise different levels of care with washing. For that reason, provided it didn't show through the front, I'd be more likely to use black or a dark color for the backing fabric. At a minimum, I'd pick the darkest color I could find that didn't show through the top.
If you reduce the stitch length to 1.6mm, you don't need a second row of stitching. You can certainly do that anyways & it will look nice. With those heavy (linen?) fabrics, I actually love the look of a triple stitch, or else a heavy weight thread. It's the one time -- other than jeans -- when I get to use that feature on my sewing machine & I love it!!!