Oh, you wonderful quilters are coming through! I intend to "pin" it with glue by making as sure as possible that everything is even and then putting a spot of glue and pushing it down with something like the end of a wooden spoon... something that will push the glue through the fabric and into the batting. I'll have time to double check before it dries.
And I intend to quilt the top according to the pattern and let it go where it goes on the back. The backing pattern is pretty complex so I don't think it will show up as tooo odd.
I hope more suggestions keep coming!