Now, just to show I have no "agenda", I thought I would throw another theory out there.
Again, this is just a theory, but it might also fit with what historical evidence is currently at hand.
Yip Man is said to have studied with Leung Bik when he went to Hong Kong for school as a teenager. He only became one of the "3 heroes of Foshan" in later adult life after he returned to China. It could be that Leung Bik was a cover story for the person that YM really studied with....someone from the HFY lineage! Maybe he used Leung Bik as a cover story because the real teacher was part of the secret societies. Maybe the Luk Sao Chi Sao rolling platform was part of HFY all along. While YM kept what he had learned in Hong Kong pretty much to himself, maybe some small things naturally came out in his Wing Chun. Maybe he shared the LuK Sao Chi Sao rolling platform with YKS rather than the other way around! Maybe Sum Nun told the story that YKS shared it with YM to save face for his teacher since YKS was technically YM's senior.
Then an aging Yip Man is back in Hong Kong. It could be that he did not completely learn the HFY method and had even forgotten some of it by the time he is teaching in Hong Kong. But he decides to share what he remembers with William Cheung. Then William Cheung (or possibly Yip Man himself) fills in the gaps by combining it with YM WCK. So TWC and HFY share some strong similarities, but are also quite different.
If a lineage emerged from China (as the Snake Crane system recently came to public awareness) that was essentially the same or a variation on Garrett Gee's system, then this would certainly strengthen this theory. But right now I would say it is less plausible than what we have already been discussing.