Been reading. Ten Desires is totally exploding with historical and cultural references. I read about the motive person of Seiga, and I'm just fascinated at how much ZUN can do.
By the way, Seiga is officially one of the most beautiful women in Touhou. At least she's the only one who is not just based on but is an figure in history or folk tales, and is actually mentioned to be stunningly beautiful in the folk tales.
ZUN really went all out when designing the Ten Desires characters. To talk about Seiga, she's based on Huò qīng é (which is the original Chinese pronunciation of Seiga's name) who is described as having a father who was fascinated with Taoism and went to the mountain to train, which is exactly what Seiga's backstory says. (Original passage: "同里有武評事者,好道,入山不返。有女青娥,年十四,美異常倫。") The name of his daughter was Qīng é, an abnormally beautiful maiden at fourteen years old.
Then here's the part about Seiga's family name. As spoken in the passage above, her original family name was different (武氏), but her husband's family name was Huò. This means Seiga changed her surname to that of her husband, which was considered equivalent as abandoning one's own parents in China during that time. Highly fitting for the twisted hermit Seiga is, isn't it?
Her name as a hermit, Seiga Nyan-nyan, isn't the same person as Huò qīng é, but ZUN seems to have made them the same for making Seiga. Seiga Nyan-nyan is also a figure mentioned in Taoism.