I checked a 古典 dictionary for あぢきなし; their etymology said it's from あづきなし, which is あ (sound variation of the わ in 分ける or 分かれる) + 付き + 無し, meaning unable to tell the difference or not distinguishing, which led to the other meanings by association.
This post has the 余釈 for the sentence. I put in links to Wikipedia articles for the Chinese history figures mentioned.
Sairyusho (1510): This is drawing from examples of the government falling into chaos from
the love of King Zhou of Shang for Daji, or King You of Zhou for Bao Si.
Kogetsusho shisho (1673): This is the example of Xuanzong's love allowing for the An Lushan rebellion to occur.
Kacho yosei (1472): The Kiritsubo Emperor survives the Kiritsubo Intimate; this is similar
to Xuanzong being separated from Yang Guifei. This whole chapter is written based
on the Song of Neverending Sorrow, and so first the text says that the "example of Yang Guifei should be brought out." The author's craft is excellent.
岷 云々 前の「もろこしにもかかる事」といへるとは別段と見るべし。花鳥には一つの心に注せらるるか云々
Mingo nisso (1598): (first part omitted) The "In China too, these things" sentence is talking about other examples, although the Kacho yosei seems to say they are the same.
The (somewhat trivial) issue is whether the "In China too" included Yang Guifei or not, something that divided the early commentaries.
You should view the "In China too" line as including the Yang Guifei example. Now
then, it doesn't even need to be said that this chapter is based on the
Song of Neverending Sorrow. Therefore if you don't know
the entire text of that poem, it's hard to understand the excellence of
this chapter. So even though it's troublesome, I will give the entire
text from Bai Juyi's works here.
Next, there is a Legend of the Song of Neverending Sorrow
by Chin Ko. This records the story of Xuanzhong and Yang Guifei from start
to finish. It is long, so I will include only an excerpt showing the
main point here, and without annotations. You should see the main text
Hagiwara does indeed include the entire Song as well as excerpts making up about a fourth of the Legend (a prose version included in the complete works of Bai Juyi). Initially I thought I would skip over this but I have to read these for my dissertation anyway, and it better represents Hagiwara's intentions to include it. There's already a good annotated translation on Wikisource, although the Legend doesn't seem to be translated anywhere. So it might be useless duplication to do it here too, but I suppose that's OK.
Also, I'm leaving Japan at the end of March and I'm presenting at a conference in a month so my free time will be a bit more limited until April. I'll still try to get a post out every 7-10 days or so but things could be a bit slower during that period. Thanks for reading!