Love the quilt!! Here's what I would do if this were mine.
Take off the outer borders. Measure across the middle verticle, cut those borders to that size, pin them well, then sew on.
Measure across the middle horizontal, cut those borders to that size, pin them wel l, then sew on.
Here's what I mean by pinning. Find the middle and quarter marks on both the border and the quilt body. Pin at those places, then find the 1/2 marks between each of those places. Keep going until you are well pinned. This will distribute any fullness from the bias. When you sew, put the body of the quilt on the underside of your seam. This will let your feed dogs feed the excess fabric as you go. Take a deep breath this is fixable!!!
This will stabilize your bias, give you a nice square quilt.
When you get ready to quilt this, lay the quilt flat on the floor, tape it square, then put some spray starch on the full areas. They will ease in to where they belong.
PM me if you have more questions! Shel