clarify "bolted" - what are the screws holding the tie-down rings to the cargo floor?
FWIW, I am sure the seat belt attachment bolts (probably the hardest points in the whole D1 body) are nowhere close to 23kN breaking strength. Whatever you've used for tie-down rings seems to be way undersized compared to the seat belt mounts.
Each tie-down ring is held to the floor by two bolts. Each bolt is backed with a rubber washer, a fender washer, and a nyloc nut. I forget the sizes offhand, maybe M8. They definitely are nowhere near 23kN, I don't think the tie-down ring is even rated.
I took the Disco on a 3-day camping trip over the weekend (and interestingly didn't use the shelf, as my aging dog is now having balance issues and stayed behind, but that's a different story). I did use the two 10-gallon totes around which I had originally designed the shelf: kitchen gear in one, dry food goods in the other. I estimate they weighed about 20 pounds combined, maybe 25. Between the pair of tie-down rings and the seatbelt anchor I think the shelf will stay put in a collision, rollover, or large bump. The weak points in the system are likely the D-rings mounted to the frame of the shelf. I did well enough in AP physics to be able to figure out the resulting forces in different scenarios given that the mass is known and I can make assumptions about velocity and time. It would be an interesting exercise.
I agree with you about the likely strength of the seatbelt anchors. I recall from my guiding days that the human body can withstand something like 9-10kN of force, so 23 would be overengineered since car manufacturers are not designing seatbelt anchors to be climbing anchors.