Your question is interesting but there is no real answer.
There was no dummy form originally. It was a training aid that some smart folks developed a curriculum around. We can even today estabilsh a core set of moves going back to the Red Boats. However different versions of the dummy arose in different families once the art spread from the boats.
I think you're pretty much on the mark here. I personally don't think you can prove any of the various histories proposed by different WT/WC lineages before the mid 1800's. And that is after the red junks of the travelling opera troupes figure into most of the stories. What can be seen is that there were various different versions of the forms going back to that time. I also agree that it is futile to seek out an "original" version.
We do know that dummies pre-date Wing Chun by centuries, and that at some point, earlier dummies were altered and adapted to be used in the WC system. The oldest dummy designs were simply posts set into the ground with three arms and a leg. The "live" or springy dummy mounted on a frame came later. And of course the form (or forms) developed over time too. Some became increasingly complex, and others, like Grandmaster Yip's were trimmed down and simplified. But we can never know for sure who were the authors or major contributors to the forms, although we can safely guess that the form was being practiced by the time of Leung Jan, based on the lineages that practice it today, as well as on the testimony of Grandmaster Yip.
I believe that Yip did two things and you can see this by looking at the dummy form he taught is Foshan compared to the dummy forms he taught in HK.
Hunt, you are a better scholar than I. When I look at the picture sequences of forms practiced by other lineages, such as some of the branches in Fatshan (Fo'shan), My mind goes blank. I can observe basic similarities and differences, but without actually seeing the forms performed... or better, being taught them... I really don't get it.
My last teacher liked Yips form because of the simplification even though the form lacked several concepts and sections when compared to his families form.
Same with my old teacher. Any idiot can tack on extra movements, but to simplify without losing content is what Wing Chun is all about. In fact my first Sifu believes that Grandmaster Yip was too respectful of tradition to
deliberately drop any movements. Instead, he believes that Grandmaster Yip may have actually become a bit rusty or forgetful about some parts of the forms during the troubled periods of his life when he was not teaching.
Then, when he began teaching again in Hong Kong,
he had to reconstruct the forms. Of course he remembered the most useful stuff very well. But perhaps he let some of the repetitive and purely "traditional" stuff slip away. The end result, ironically, was a net
improvement in the forms, at least by the Wing Chun standards of efficiency and simplicity.
I do not know if this is true. But my sifu studied with Grandmaster Yip and believes it may be so. An interesting idea at any rate.