THAT is one beautiful quilt.
Sharon Schambers says to spray starch the backing. It's worth a try.
One of the ladies at my guild goes through an elaborate process to sandwich her quilts that may work for you:
She tapes toothpicks to the table in a perfect 12-3-6-9 o'clock pattern. When she puts her backing down, she aligns crucial parts of the backing with the tooth-picks. Then she smooths out the backing and clips it to the table with those clips that hold down plastic tablecloths. then she centers the batting and pats it smooth (no tugging) and she aligns the top on the tooth picks as well.
She uses pins no further than a palm-width apart, but you may want to use Sharon Schamber's herringbone basting stitch.
It takes a long time but I think your quilt is worth the extra effort.