I have always liked to use fabric that could substitute as a throw for the back. It's an easy way to make a two sided quilt. On the other hand, this quilt is so "busy" you might chose to stick with something that blends or augments the top. I think green with the dots, or that blue, would make a great backing.
As far as the distance for cross hatching, first of all check your batting to see how closely it has to be quilted. For a whole quilt, I tend to cross hatch with at least 2 inches or more between the lines. However, I do find that to be a bit boring (to do and to look at) so I'll pull out sections I want to highlight and either just use close lines, or a much smaller cross hatch. That will add some variety to your FMQ. On the other hand, your quilt is bright enough on its own to use a wide cross hatch and let it "disappear" into the quilt.
You've done a great job! Very impressive!