Oh Jan..that is beautiful and I can sure see why you would want to use that back...its fantastic! I can't help you out with anything that hasnt' already been said...my last quilt had a straight line on the back..I did all I could...it ended up looking as crooked as a dog's hind leg. :evil:
I hope you can get it worked out though...I do like the basting spray and I might use a combination of that and pinning. I do use the toothpick method to line things up too. I think with mine..I just didn't check the back often enough while I was quilting it..(I tend to do that)..Good luck!