I’ve been digging around in Ancestry.com to see if I could find anything interesting. And two things jump out at me that have to immediately discount the “The Roffs and the Vennums didn’t know each other” story. The Wonder events started in 1878. In 1880, the census records for the township Watseka was in show fewer than 1,350 people living in the township. More than that, Lurancy Vennum has an older brother the same age as the youngest brother I could find for Mary Roff (just doing a very quick search out of curiosity).
In fact, that youngest brother was only three or four years older than Lurancy.
Which means this wasn’t a matter of a decade’s distance between one family losing their daughter and a family full of strangers becoming possessed by her. Roff’s death was what happened to the sister of a kid Mary and her siblings went to school with. It would be more implausible that she didn’t know of the circumstances of Roff’s life.
Which is fine.
It just means that the assumptions the outsiders who came in and wrote about it 30 years after the fact (without being able to interview Vennum) were wrong, and wrong in ways that make their declarations about what’s going on untrustworthy.
But whatever happened, it did keep Vennum out of the asylum, so that’s good.