Honestly, I take that story I relayed with a "grain of salt." I think Wing Chun has had the "3 form" version of empty hands for a long time. Leung Jan passed this down to Chan Wah Shun, and his classmate Fok Bo Chuen passed this down to Yuen Kay Shan, strongly suggesting that this is how it was taught by Wong Wah Bo. But Weng Chun has never had this format. If there is any grain of truth to that story, it is probably that the biomechanics and applications were likely very similar if not the same, even if the curriculum and teaching format was different. So that still leaves room for "divergent" development or "drift" even during the Red Boat stage of history or before. And I am sure we would see "drift" across all of the lineages over time as you said. Most of these guys knew more than one system of kung fu. If you have guys that have been doing Hung Kuen since childhood that then convert to Wx Chun, their background in Hung Kuen can't help but influence how they do their Chun. This was certainly a factor in Weng Chun down the years! Sum Nung studied a different version of Wing Chun before he became Yuen Kay Shan's student. He even incorporated some of it into the curriculum when he began to teach. So his prior studies certainly influenced what he taught later. Chan Wah Shun's son taught something totally different from what most people would identify as Wing Chun or Weng Chun because he added in a lot of stuff from different styles. So really, I don't think there is any such animal as "original" Wing Chun still around. Every living example has changed somewhat with each generation that passed it on.
And back then there was no video recording to prove what previous generations had practiced. So you can have guys like Wong Shun Leung stating that he was teaching just what Ip Man taught him. And we don't know how to interpret that. He wouldn't necessarily mean he was teaching the EXACT same curriculum and EXACT same biomechanics, etc. Wing Chun is one of those systems were you are expected to adapt things to yourself and not necessarily just copy your teacher. So knowing this, maybe WSL was doing what Ip Man taught him, even if he had modified, stream-lined, improved (!) etc what he was passing on. Same would have gone from prior generations. If someone asked Ip Man, he probably told them he was doing exactly what he learned from his teacher. But we know he changed things over the years. That doesn't make either of them liars! They likely just didn't mean what we think they meant! ;-)
Agreed. I have said again and again, "my WC is not your WC. We all bring ourselves to our art." Previous training will inevitably be part of this.
As for the last I think, to an extent, "I teach what my Sifu taught me" can be boiled down to the following, for the modest student.
1. Respect. If you do something different it can be perceived by some that you are saying "My Sifu was wrong." There are many other reasons to explain a change of course but to avoid even having to engage in that conversation you may simply say "I teach what I was taught."
2. Reality. A single Martial Art is VAST in scope. Techniques, foundational principles, strategy. Often the difference between Master and Student is priorities.
Then you combine how we all bring ourselves to our art. I think, as you raised his name, WSL is the perfect example. WSL was first, according to what we a told, someone training in Western Boxing. This would help explain why he focuses of the punch, where others may prefer the palm strike.
Then we have the video I posted of DP speaking about the mook jong. DP says that WSL changed the order, putting the techniques he felt most relevant, based on his challenge fight experience, first. He didn't change the techniques though, only the order and that was based on how HE fought the challenge matches. However nothing he teaches, technique wise, is something YM didn't teach him, WSL simply prioritizes in his own way.
