OK, after looking really hard - needed your hint as to where to look, I see your "disaster". If the slight angle really bugs you, why not put on 2 borders on that are wonky...like one starts out 2" at the top and finishes at 1" at the bottom then put a strip that is 3" wide and trim it to square, which will make it different widths at the top and the bottom - the wonky there will mask the wee bit of wonky in the quilt.
OR you could heavily starch the piece and pin it square and let it dry...that covers a multitude of sins by stretching and shrinking to make it square.