This quote from bliss is a good reflection of my current theory about Dugu Qiubai and Dugu 9 Swords:
Yes, this is exactly what I think. Using Ren Wo Xing's words, I believe that Dugu 9 Swords is the synthesis of all the stages (maybe even the No Sword stage). I also believe that Dugu 9 Swords was not used by Dugu Qiubai to compete, because it would've been developed later. Dugu 9 Swords is so comprehensive that I find it hard to believe it could have been invented by a young Dugu Qiubai who hadn't reached the Wooden Sword stage yet, because Dugu 9 Swords already somewhat encompasses the Wooden Sword.
The trouble lies in Feng Qingyang's words. He said: "When senior master Dugu took on the entire Martial World with this set of Dugu Nine Swords many years ago" This literally means that Dugu Qiubai used Dugu 9 Swords to compete with the heroes of the martial world, which would contradict with the theory that he didn't invent Dugu 9 Swords until he retired.