YKS and YCW did know the entire system. Yip Man developed his Dummy form and evolved it in HK.
It my understanding, not factual - but consistently upheld by documents and 1st hand account, Yip did learn various things from the Yuen's. Aside from luk sao/chi sao, who knows.
Possible that Yip said don't perform dummy in front of Yuen. NO ONE knows this to be the true reason. I know both of these forms and they are DRASTICALLY different. Different jings, methods and choreography. Also, the YKS dummy is much longer and more intricate than YM. Not saying one is better than the other, just that they are very different. as are the knives.
I have studied and completed BOTH YM and YKS systems, I teach both. I can say first hand that ALL WCK is similar. The differences come from methodology, jings, center, issuance and shapes. I have my personal preference of which is better, but that is for me. BOTH have their place(s). being able to adapt and move between the two methods has served me well.
Here is a clip of Kwok Wan Ping, my line of YKS. Keep in mind, the gross motor skills and methods are correct. its just not as exact and tight as I'm sure it once was.
Not trying to FLAME. Just give more insight since I'm from the line.