Always back everything up. Never trust One piece of gear. slings are very strong. all but the most unusual cases do slings fail. Rope, rope drag, belayer, rope slide through belay device, you and the occasional gear pop all absorbs impact. Its like catching an egg you just dont stick you hand out and try to catch it, it will break. you reduce the impact by lowering you hand with the egg to catch it, thats sort of how the system works. But every situation is different and sometimes you just got to take what you can get.
I reasoned that 2 parallel strands of 16mm is ~ like 1 strand of 1" tubular for k (but not breaking strength); since there is a square root dependence on k, a bit of error is reasonable.
I calculate about 10.5 kN for the nylon sling with FF1, and 14.5 kN for FF2; that's pretty close to their results, considering that the webbing k is uncertain, and there is probably greater potential energy involved than just that implied by the sling length, as the center of mass of the weight is displaced at the bottom of the fall.
I use static webbing a lot for handlines, and in a pinch, we've used 15mm webbing for belays and raps. I am happy to have a physics concept turned into something that sticks in the brain.
It's good to realize what the high modulus of static webbing or ropesactually means in real life. (Some rap ropes are actually more than 5 times stiffer than 1" webbing.) When you take a fall on this stuff, the energy must go somewhere. If the static material is connected stiffly to a fixed point and your harness, it can snap with a surprisingly small fall; or your body will absorb a few 10s of kN, probably not in a good way.