Who cares about the backing fabric? I'm not saying use some crappy fugly fabric, but just find another fabric that goes with the backing fabric. The only time anyone sees the back is when they're lying under it and they have, what, 15 inches turned back at the top. Don't even worry about that.
Most of my backs are pieced, and I only ever worry about the top 20" showing.
As to the problem with the corners, that is a pretty common with the quilt.
Since there are SO many 9-patches, that unless you sew a PERFECT seam allowance throughout the entire quilt, and your cutting is to the thread, you're going to have distortions. Also, the blocks are set on point, the rows are very long, and the setting triangles are QST, so things do get stretched out a bit.
Another big mistake made is that the solid squares are cut at the beginning. So even if the 9 patches were done and they all turn out to be 4 3/8 or 4 1/16 (they're supposed to be 4 1/4) the entire quilt is matching the two sizes together. That's a lot of easing that will also cause problems.
When I was teaching this quilt at a LQS, I ended up having 2 lucite square rulers cut for 4.25" and 4" (this was before they came out with the 4) so the students could trim up all the 9 patches - if they were too small. Painful, yes. But then when they cut the squares to whatever THAT measurement was, the quilt went together very easily.
One of the things I discovered (around my 4th one :roll: :roll: ) was to make oversize setting triangles, so the previous blocks float. That way, you don't have to worry about having the a less than perfect 1/4 left over, or having it distort during quilting, and then having it get trimmed off in the squaring. And I like the look better, especially if you want to add more of that last fabric as a border.
Sorry to go off on a tangent. I just really, really enjoy this pattern.
In your case, since you want it bigger anyway, I woould add borders (3"? or more - that should add more than enough for you) around the whole thing, using that same fabric.
You don't want to mess with 1" borders - because I'm sure the quilt is not square and you'll probably lose some it in the squaring up anyway. This way you can preserve some of the points, and the squaring/straightening will be less noticeable on the border fabric.