Border question - already answered
Flannel - Yes go for it, I love flannel on back. However climate might be something to consider
Quilting - panto or all over stippling looks great. I have used one called Deb's Swirls and another called Contempo(squares) which have both come out nice. Have you LA lay out a couple of different colors of thread or you can do that at home with your thread if you have several colors and see what you think. I will tell you that the thread does not show as much as you would think. With the texture of the knit fabric it seems to almost sink in some so you see the design more than the thread