I am wondering if the Prince of San Donato line truly has expired. There may be a ray of hope in the form of the Dournoff family (spelling?). I have the document granting the Tuscan title of Prince of San Donato to Anatole N. Demidoff on his marriage to Princess Mathilde Bonaparte in Florence dating to the 1840's. There is a reference for the title, Prince of San Donato, to also transmit down the family of the Dournoffs (spelling?). Anatole's father, Nicholas Demidoff, had a sister, Maria, that married Peter Dournoff so this family also are cited as legitmate heirs to the title of Prince of San Donato. If this is of interest to you then I am fine to return with more accurate detail, references, and spelling after I retrieve the historical records.
Kind regards, Alexandre