I recently read Things Fall Apart by Chinua Achebe, and found its detailed portrayal of historical Igbo (aka Ibo) culture very interesting. My understanding is that it's a reasonably close approximation of actual Igbo culture, but does mix in elements of other similar cultures plus a little artistic license (presumably when it was written in 1958, some of the relevant cultural practices still active in the 1800s when the novel is set would have died out and would have been hard to research).
The culture depicted, that of the Igbo people, is similar to that of Achebe's birthplace of Ogidi, where Igbo-speaking people lived together in groups of independent villages ruled by titled elders. The customs described in the novel mirror those of the actual Onitsha people, who lived near Ogidi, and with whom Achebe was familiar.
I'd like to know if possible which specific practices are true of the historical context and which, if any, are known to be not true of it - for example, which elements were true of Onitsha people but not Igbo, or true of the 1950s but not the 1890s, or which if any were invented for the story. Particularly around the organisation of society, things like the roles of the masked elders who act as spirits (which is similar but different to many current traditions across West Africa).
I've looked at a few sources, but I'm having trouble picking any actual facts out of the various literary criticism essays and reviews on the topic, for example this rambling essay and the Cliffs notes talk around the question of historical accuracy but don't seem to give any actual facts.