I find it very irresponsible of Duan Yu for not passing down 6MSJ in a manual. 6MSJ is part of his family's inheritance, it was his duty to preserve it.
I can understand if he doesn't meticulously record BeiMing ShenGong and LBWB since those were the personal treasures that he found. But 6MSJ is like a family heirloom, it should have been handed down to the next generation.
Now, even if Yideng had access to 6MSJ, there is no way that he could have used it at the first HuaShan meeting, as his inner power was not sufficient to master it. He had yet to master 1 Yang Zhi by that time.
After he became a monk, although he may not covet the title of strongest in wulin anymore, I think he would still try to practice 6MSJ if he had access to it. Even the retired Duan royal monks during the DGSD era studied 6MSJ, although none has mastered it.
At what age do you guys think Yideng had enough inner power to master 6MSJ? At the end of LOCH, where he was around his mid 50s? Or maybe in ROCH, at the time of his duel with Golden Wheel monk, when he was near 90?
I believe Jiumozhi, who was in his late 40s, managed to master one stroke of 6MSJ, the one with the pinkie. He had great inner power on par with Xiao YuanShan, maybe he could have mastered all 6 strokes if he didn't run into health problems + get drained by Duan Yu. When Jiumozhi fought the 6MSJ formation of the Duan royal monk, his inner power had not peaked yet, since he had yet to study the 7 out of 8 manuals of Xiao Yao inner power (the same kind Li QuShui used). Yet this pre-peak JiuMozhi fought the 6MSJ formation to a stalemate, and was confident he would win in a 2nd clash, so I think given enough time, Jiumozhi could have become proficient with all 6 strokes of 6MSJ.
Some say that Jiumozhi was around the same level as Xiao YuanShan, Murong Bo, Yang Guo and Guo Jing, so he was (slightly? moderately?) stronger than 90 years old Yideng though.
If Yideng had access to 6MSJ, could he have become the #1 fighter in the world? Could he fully use all 6 swords like Duan Yu, or perhaps only partial mastery like Jiumozhi? Would OuYang Feng covet 6MSJ as much as the masters of the Northern Song era coveted it? Let's not forget that 6MSJ was THE most prized manual under heaven during that period.