It's possible James remarried to a Mary Wilson by mid 1850's as there are a number of children born in Hawick to them so it's possible he was living there when Jean/Janet died.
So you're looking for a Jean/Janet Deans/Rutherford 1845-55.
Have you ascertained where the plaque may have come from ?
Are there any derelict/demolished Churches/Chapels in the area that it may have been removed from ?