I think I might have found out what happened to the Elizabeth/Isabella Thompson that I was searching for. A clue from the research above, especially the marriage record of John Thompson and Elizabeth McDuf/Leduc, lead me to what appears to be her death record.
From LDS FamilySearch - Ohio Deaths 1908-1953, I found the death record for Isabella Keir (wd) who d. 24 Mar 1938 in Lucas, Ohio. The record shows:
birth: 7 Jun 1862, St. Eugene, Ont., Canada
mother: Elizabeth Mcduf
spouse: Thomas A. Keir
So, it looks like Isabella/Elizabeth is the wrong Elizabeth Thompson I have been looking for.
If anyone sees this differently, please let me know.