I never prewash my batting, nor any of my fabrics. I might do a color test on a suspected bleeder. There is no one right answer for that question.
Free motion quilting takes a lot, and I mean, a lot of practice. But although I still consider myself on the low end of the intermediate skill level, I enjoy it a lot. I still watch a lot of videos and practice. The YouTube videos make it look so easy because those folks have not shown you all their hours of practice. Check out LeahDay.com and Angela Walters. They offer a lot of free demos and tutorials.