why not press the seams in the sashings/cornerstones so they nest? seems like that would be the easiest most accurate way to go- if they are sewn down to one side you just clip them to the stitching line so you can fold them in the other direction.
when i paper piece and have 2 pieces come together where the seams are in the same direction i poke a pin straight into the point of the top piece- then line up and poke straight into the bottom piece making sure they are perfectly joined- holding the pin straight up & down i then carefully pin the 2 pieces together on each side of the straight in pin- that technique may work. place a pin directly next to the seam- then line up with the bottom piece seam- and pin on each side of it then you can remove the center pin