"Roundsling question: The datasheets specifically forbid knots of any type (i.e. to strop two slings, or shorten a sling), under any circumstances, regardless of the load, don't even de-rate it, just no.... why?"
Here is a video I took.
The slings have a WLL of 2 tons.
By the way ... here is a video where I tested industrial slings in a girth hitch. It were 2 destroyed slings from a previous test, each knotted together.
Again ... the knot had no influence.
Rules (according to my tests, not statistically approved):
1. If you can open a knot after a sling being loaded ... relax, it was safe.
2. If the force in the sling is divided in two branches, a knot is no problem, because it has about 50% reduction.
Only if the knot is directly loaded, it reduces the breaking load.