well, considering minor oni, it's difficult to say. In japanese folklore most oni are probably male, with some notable exceptions. Anyway, the title of this chapter is "鬼女" (kijo), which literally means "female oni", and in fact it indicates young female oni (old female oni are caller...
Yeah, the doctor was suspicious, not immediately, but definitely later. I thought he could have been the murderer, but something told me it couldn't be so easy.
I think it's mostly satirical. Female mangaka tend to criticise sexist mentality way more than than their male colleagues, even though in a more subtle way, while giving the reader what they would like to read, of course.