Beautiful quilt! I am probably not the best at answering your questions, but I will give it a shot. I cut my batting about six inches bigger than my quilt...3 inches per side. I make the back of my quilt that size also. It is probably a bit bigger than I need to cut it, but it works for me.
If you are going to follow the pattern, stitch in the ditch, you won't need to mark your pattern.
Oh, I just looked at the picture again. You already have it pinned. Good for you.
One thing I learned the hard way is that q quilt can get a bit heavy while quilting, and gravity wants to pull it away from the sewing machine. I now place my ironing board perpendicular to the sewing machine and let it hold up part of the weight of the quilt.
It looks like you are 'good to go." I find the machine quilting very satisfying. I hope you do too. (I have been quilting 2 years....sort of a confident beginner...)