Obviously not much to add to this, and I thank you for compiling it. Just thought it would be interesting to link this part of a lecture from Milton Friedman where he specifically mentions De Beers as one of two monopolies which he cannot explain (i.e. come up with the evidence of government intervention that allowed them to remain)…although he defers to his colleague George Stigler who may well have been able to provide at least some of the info presented here had he been present there.
Also relevant is this thread: