I like to starch my backing fabric heavily before layering. Starch keeps the backing from puckering during machine quilting.
How far apart are your pins? They should be no more than 4 inches or so from each other.
If you are stitching in the ditch on long lines, I don't think the hoop will help you at all and might even make things worse. This is because you would have to re-hoop every 10 inches or so (depending on how big a hoop fits your sewing machine harp), and the re-hooping would have to be done with the presser foot in the middle. So much handling of the quilt sandwich is likely to introduce more distortion than if you didn't hoop.
A walking foot would likely help a lot, especially if you have no starched the backing fabric. I think the generic ones cost around $25.