The lack of squareness after quilting is probably because you did all diagonal lines. Did you alternate the direction in which you sewed the lines? If not, next time alternate direction every time you sew and that should help. It's amazing that, even with spray basting and starching, the sandwich probably stretched along the quilting lines.
Edit: Come to think of it, alternating direction probably wouldn't help. If all the lines were running in the same direction (no matter if you alternated sewing direction or not), my bet is the quilt is longer at the opposite corners of where the diagonal lines started and ended. I'm not sure how to prevent that from happening when sewing on the diagonal -- unless, perhaps, you sew cross-hatching on the diagonal and are careful to sew just one line at a time for each direction. I think that would be the only way you could keep the corners even and the quilt square.
Last edited by Prism99; 12-31-2017 at 07:59 PM.