Dealing with the 'lumpy center': I've just made about 40 of these lovely blocks and have found a nice trick to make the centers where the seams meet lie down flat. When you finish the third seam, take your seam ripper and undo the two or three stitches above the main seam, at the intersection. Do this on both sides. These two or three stitches are no longer necessary, and when you take them out, you'll be able to flatten the center intersection into a neat square.
I wish I could post a picture or diagram of this, because it's far easier to do than to describe. It takes only a few seconds and gives you a nice flat background.