Rodney and Olivia, it's beautiful; the obvious love and care put into it is all she will see and feel in her soul. Also even experienced patch-workers end up with issues like that. I always tell new quilters that undoing and redoing is just part of the process, and it helps us learn the pre-sewing things we need to do to avoid it.
What I myself would do in this instance is to distribute the 1/2" correction across the width of the end block...
* use a seam ripper to undo the sash seam only enough to remove the one end block
* use a rotary cutter and mat to trim 1/8" off the left and right sides of that removed block
* between the blocks 4-patch rows, resew each of the two vertical seams 1/8" inward from the previous seam
* sew the block back into the quilt row to see that the extra 1/2" has disappeared and that the long perimeter sash strip will now line up.
* after sewing on the perimeter sashing, measure the quilt-top width in three or four spots to obtain an average width, trim the quilt-top width to match that average measurement, then repeat this step for the quilt-top length to square up the entire quilt-top. This will make any unevenness invisible to the eye.