Yes, very interesting post.
With regards to the minimal speed margin for flaps, some B742s had their max weights increased to that of the B743s, 377842Kg. This put the min clean speed at precisely the max F1 speed for the 200 when at max structual. Being a freighter, that was often. To resolve that issue the max F1 speed was increased by 5k and that therefore became the margin. At the time our fleet was mainly 300s with a couple of 200s and then the company purchased 3 200s (while waiting for the 744F to be delivered) to be used as freighters and had the weights increased on those 3 to that of the 300s.