Yeah, the complete and utter change of appearance as the sole cure for a barkingly mad poison definitely causes me suspension-of-disbelief problems, but barkingly mad, unique-to-this-particular-story poisons are indeed a genre thing (and even in The Story of Minglan, which is relatively restrained when it comes to genre elements, there is a sub-plot involving a poison which, unlike all other poisons (apparently) cannot be detected by sticking a silver needle in it. But I'm afraid there is worse to come (I say this because you say that you "think" you might be hitting the WTF bits, and believe me, when you hit the truly WTF bit, you will know). When you reach that ooint, I suggest you read the Langya Hall Diaries for comfort (here: https://qi-lang.tumblr.com/post/145919011060/lydiaries1), I found them a great help.