Hello SPQRRR, thanks bunch for your kudos.
I am not the right person to reply to this, because I am sure there are many more people here knowing more about LOCH and ROCH, but let me try. As far as I know, that woman in yellow robe is the descendant of Yang Guo, maybe his granddaughter. She said something about coming from Mount Zhong-nan, where Xiao LongNü used to live. In the book, she was described to be in her late twenties or early thirties, but not married yet judging from the clothes she was wearing.
As to why she could use Jiu Ying Bai Gu Jang, hmm....I am puzzled as well, the skill used to be in Huang Rong's, Guo Jing's and Guo Xiang's line because it was written in the book stolen by Huang YaoShi's 2 pupils. HSDS described that though ZZR's Jiu Ying Bai Gu Jang is evil, hers is more "open" and "clean", not evil, and that ZZR was in a hurry, so she did not really master the basics first but jumped to learning this skill which is more lethal and evil. There is a possibility that the yellow-robed woman may have learned it from a living person, not from the Jiu Ying Zheng Jing.
Sorry for not being of much help.