When I took a class on OBW, we were told not to pre-wash the fabric when makng a OBW because you are trying to find exact landmarks on different layers. BTW, I did it with almost the same fabric and it is very pretty.
Mine is a wallhanging so I will never wash it so I dont have an answer about the backing/batting issue but I always thought shrinkage was about 3-5% on name brand fabrics.