The 'photograph' on the shelf

Since it's been two years since you asked you might not care anymore but the photograph the framing story mom keeps looking at is meant to have been taken years after the events in the central story. We are to infer that Betsy and her professor did get married later. The photo is typical of pictures of married couples. (Presumably, the professor wanted Betsy to reach a more appropriate age and to have a chance for them to get to know each other outside of a teacher/pupil relationship. He says he doesn't want to be a last resort so we can probably assume that once it happened it was a mutually consenting marriage.)

The manuscript found in the attic is dated 1848. Prof and Betsy had plenty of time to have a portrait taken while still youngish and fresh faced. I think we are meant to understand they then lived in the Bell house, where frame story mim and daughter now live.