Details of what went wrong are in my original post here.
I tried to quilt this no less than six times, but didn't have much luck. I know now that it's because I was using really slippy high loft batting and bad quality thread. Eventually, I had to admit that I didn't like this quilt very much, and it has been sitting in my sewing room unfinished for six months. This week, I resolved to finish it.
I used the zippling design from this week's Free Motion Quilting Project exercise to finally quilt this properly. Then, I pulled out all of the really bad stitching in the ditch that was left over from my last attempt. The zippling was fun and looks good, definitely one I will use again. It's easier when there are no curves to worry about!
I also stitched around the turtles to give them more pop, and they did become really nice and puffy.