A mistake is proof that someone tried to accomplish something.
Your quilt is really great! A very complex pattern in the block that will only attract more attention to the center than the edges, where I have found ... the final darts end up under my binding. If I were in your place, I would definitely pin the binding, easing in the "flow" of the border to evenly space the ruffling effect.
I have come to appreciate the advice about measuring the center of the quilt both ways and making the border be that size. It is our final chance to control the outcome, and well worth the extra time it takes.
I completed a "Victory" quilt (elenor burns) making myself absolutely follow all of the directions - - she made a believer out of me about knowing the size of the border ... my borders were spot on.
YOu have done a lot of good work on your quilt. Celebrate your accomplishment, learn from the not so perfect parts, and move on. Nothing ever will get anything but better.
Not to be a party pooper, but ... you could take the border off and do the measuring, etc ... there is always that option <g>.