I love the black combined with the florals. Looks very pretty!
I do like cotton batting, but make sure you pay attention to what the package says as far as maximum distance between quilting lines.
As far as the corners of your squares not matching up, believe me, it is not nearly as bad as the corners in my first quilt, which were off by nearly half an inch at times. But I tied it, so I put ties over the corners and it hid the worst of it.
And also, my first machine quilted quilt, I cut backing equal to the batting and top, and when I was finished, it had "shrunk" and was up to 1" smaller in places than the top:hunf:, so buying enough batting and backing materials is important!