In my opinion for what it's worth, your quilt is absolutely beautiful. I do not and never plan to hand quilt, hate to even do the hand sewing on a binding. I think I would complete the quilt and give it to your grandmother, then if you have time later and want to redo that corner, then do it. I am sure your grandmother will be so happy and pleased that you love her enough to take the time to make her such a beautiful quilt, and by hand no less, that even if she is a quilter, she will not care if she even notices the mistake. When I find a mistake in my quilting, it drives me nuts until I correct it, but there are times I have had to let it go, due to time restraints or something else.
I had the same Band Director from 5th grade through High School, and when some one would make a mistake, - he always noticed. Instead of getting angry and fussing at that person he would tell us to try our best, and to correct our mistake because it would usually be 'heard' or 'seen' if we were marching on the field, - but to also remember that "Only one Person was ever perfect, and they crucified Him". Strange how a comment that was made so long long ago has been remembered for so long, and whenever anyone makes a mistake and calls it to my attention, I always think of what Mr. Rose would say. I can even picture him standing in the band hall and almost hear him in my mind so many years later.