Well, the longer I spent removing stitches, the more I didn't like the way the back was aligning with the front. So I ended up unpinning the entire quilt.
For the re-pinning of the quilt, I adapted the method of pinning that Harriet Hargrave teaches in Heirloom Machine quilting.
I first wrapped coffee stirrers in blue painter's tape to thicken them, then carefully measured the center of the board in both directions and added more tape to secure the ridges.
I folded the batting in quarters and unfolded it to center over the backing. This was a little harder for me as there wasn't an easy way to confirm where the center of batting was. It did end up slightly off, but not enough to make me take the quilt sandwich apart a third time.
Starting from the center I worked my way out along the 9-patch blocks, pinning in the signature blocks as I don't plan to stitch in those, and then added pins to the sashing strips and finally on the borders. I did manage not to pin the quilt to the fabric cover on the big board, but I also had to go back and repin some pins where I didn't manage to get the pin all the way through the layers. I'll need to figure out a better surface to add next time - posterboard would probably work without adding too much bulk, and would allow me to keep my ridge intact.
Hard to see as I'm using invisible thread in the bobbin as well, but they really are straighter than the first time around. Unfortunately, I confirmed that my math logic was off, so the seams on the back sashing are not where I envisioned they would be, but they are straighter than the first try.
My stitch in the ditch accuracy is improving, as is my pinning. Off to start on the horizontal seams and then to decide how to handle the 9-patch blocks.