I've finished sewing the pieces together and as I predicted, the corners were not an exact fit by any means. It almost seemed that I ended up putting them together intuitively - or maybe I should say 'randomly' and 'fortunately'. I know I was surprised when they worked out - my method after stitching each side of the corner on and discovering gaps and misalignments was to sew it from the backside, aiming for the seam of the border, which actually worked out not too badly. This morning I sandwiched the top, and quilted it in the ditch, but only partially. The close-up photos show how I've basted (who me? This is a first - I'm more 'attached' to glue:)) the top's edge, then cut away the batting flush with the top edge and then cut the edge of the backing fabric.