Matt, I’m sorry, you’re right on the forces.
Looking at the photos again, I tied the loops backwards from my original plan, which definitely put all the ‘pinching’ force on the tie in points as opposed to the seat back as I originally intended, and that explains the delamination perpendicular to the direction of pull– the tie down is getting forced into itself as the cord slides thought he holes. In testing with the sliding loop on the seat back as I intended, the seat back took those forces just fine with no indication of future failure, but I never tested the tie down in the same manner. I wanted rotational stability, easy on/off, and minimal material… I probably shouldn’t try for all 3.
I’m doing some troubleshooting on some fabric scraps now. My wife’s boat is red fabric, and hasn’t had delamination issues… It seems to be just the blue fabric that is problematic. I’ll let you know if my test pieces behave differently after soaking for a while to replicate the repeated wet-outs. So far her perimeter line tie downs are solid, and two of mine are starting to delaminate slightly, but there’s way too many variables for that to be a good comparison.