FWIW:
Cheung Bo's son, Ah Chut, learned from Sum Nung. What he taught Mak was pretty much Sum Nung's system.
Yuen Kay-San learned double knives from Fok Bo-Chuen, however he kept Fok's set in the family and taught his students versions of Cheung Bo's set with a few movements from Fok Bo-Chuen, and Yuen Kay-San's mechanics.
Yuen Jai-Wan apparently taught more WCK to native Cantonese than native Vietnamese after he moved to Vietnam. That wasn't an uncommon thing at the time. It was also a period and place where history and lineage didn't matter as much as survival so people combined what they learned from Yuen with other kung-fu styles and a lot of hybrid variations were passed along.
Chan Wah-Shun's grandsons put a lot of their village kung-fu into their Wing Chun Kuen.
The story of Yip Man's house burning down is fairly well known in China and I've heard it from many people and seen it in print. Yuen Kay-San and his two sons helped fight the fire, and Yip Man stayed with him following the blaze.
Again, you can check New Martial Hero for the story about Kwok Fu's training at the time and Sum Nung's offer to accompany Mok Poi-On to visit Kwok to make sure it was properly documented. Suffice it to say Yip Man was a reluctant teacher at the time and it's widely known in China that Kwok and Lun Gai continued their training after Yip Man left for Hong Kong and Sum Nung moved to Guangzhou.
Reputation was everything back then and they each knew the others skills fairly well. When others weren't around, I'm sure each had their own "fish who got away" stories, but when face to face with each other fairly accurate accounts had to be relayed.
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