That's rad. My main concern with historical accuracy were these statements, Ajax:
"In late YE 43 a volcanic eruption and subsequent earthquake would cause a massive cavern collapse killing an trapping many Oni but would reveal the Oni to the surface dwellers. The Yamataian government dispatched search and rescue and aid workers."
...
"All Oni live under the surface of the western continent of Yamatai, however with the volcano eruption and subsequent earthquake Oni can be seen on the surface of Yamatai."
As you can see from Wes' history, everyone knew about Hell's inhabitants and Yamataians and inhabitants were able to come and go as they pleased. By saying oni lived in all the same places as lorrfolk, what would have happened to them would have happened to oni, too.
My fix: Maybe oni lived in specific enclaves? Maybe oni only lived at a certain depth? Either they were so deep or so insular even within Lorrfolk that the happenings of above mattered not? Maybe you have a better idea!
But coming around to your original inference, it does seem like you could be adding more by making Lorrfolk playable. Oni are like Lorrfolk but retconned in to exist. If you just made it Lorrfolk that go by the term Oni, then I could see this species lasting. Either way, they're so fucking rad. I'm stoked to see them playable however they turn.
(Also please include OOC, include wiki links, the environment section contradicts itself by saying they ALL live underground then outlines where they live above ground. And thank you for submitting. It's never easy and me coming in talking about something I love is only possible because you put the work and love in to the article in the first place. Don't let anything I say matter ilu good luck!!)