In my experience, it's not unusual for family names derived from patronymics or occupations to be formally singular, and when you can pluralize them, they often follow specific family name patterns instead of the pluralisation pattern of the underlying noun (e.g. German Schmidt / Schmitt / Schmied "smith"; if you talk about the family, you don't use the regular plural die Schmiede, but the family name plural die Schmidts / Schmitts / Schmieds). To take a patronymic as an example, the family name is Mendelsohn, not *Mendelsöhne, and if you refer to the family you pluralise the singular-based last name as die Mendelsohns (using German and not English examples here, because English has the same issue as Latin that the genitive singular is identical to the nominative plural).
Russian avoids the problem by using possessive adjective based surnames for patronymics.
Concerning the Italian names, in my copy of Lausberg's "Romanische Sprachwissenschaft" they are not listed as remnants of the old genitive, so he seems to agree that they are nominative plurals in origin.