Information Structure constraints on the realization of subject-verb agreement: the Sienese puzzle

The aim of this talk is to present some unexpected agreement patterns that are found in Sienese, an Italian dialect spoken in the Tuscan town of Siena. As opposed to Standard Italian, third person plural subjects sometimes trigger third person singular agreement morphology on the verb. The mismatch is not only affecting Number, but also Gender, which is only visible when a sentence includes a past participle of an unaccusative verb.
Gender Mismatch
 (1) É             arrivato                          la                   lettera.
be-3.Sg   arrive-Past.Part-Ms.Sg   the-Fem.Sg   letter-Fem.Sg
‘The letter arrived’.
Number Mismatch
(2) Ha              cantato   una   canzone   i                 su              amici.
have-3.Sg   sung       a        song        the-Ms.Pl   her-Ms.Pl   friends-Ms.Pl
‘Her friends sung a song’.
Gender and Number mismatch
(3) Le                 tu                   figliole                     è             venuto.
the-Fem.Pl   your-Fem.Pl   daughters-Fem.Pl   be-3.Sg   come-Past.Part.Ms.Sg.
‘Your daughters came’.
I have been looking at three factors that could possibly play a role in this context: Word Order, Information Structure, and the nature of the subjects and verbs that are found in this special configuration. Word order does not seem to directly affect the agreement pattern, for agreement mismatches arise with both SV and VS orders. There is also no asymmetry with respect to the type of subjects that are found in this configuration; both full DPs and pronouns, animate and Inanimate DPs are indeed allowed.
Information structure seems instead to play a fundamental role with respect to the realization of subject-verb agreement morphology. If the subject is marked as new information, then full agreement is replaced by some default agreement form. If the subject is instead old information, full agreement must be realized.
Subject Focalization
(4) a. Chi    ha               parlato?
who   have-3.Sg   talked-Ms.Sg
  ‘Who talked?’
b. [Le                mi    figliole]Foc            ha               parlato.
the-Fem.Pl   my   daughters-fem.Pl   have-3.Sg   talked-Ms.Sg
‘My daughters talked’.
Subject Topicalization
(5) a. Che     hanno        fatto    i                tu       genitori?
  what   have-3.Pl   done   the-Ms.Pl   your   parents-Ms.Pl
‘What did your parents do?’
b. [I                mi    genitori]Top    hanno        vinto.
the-Ms.Pl   my   parents-Ms.Pl   have-3.Pl   won
‘My parents won’.
I believe that there are at least two ways to approach these data: a more semantic and a more syntactic one. This is not surprising afterall because the data presented clearly seem to be the result of the interplay of Syntax and Semantics. I will explore both options during the talk and try to understand which one might lead to the simplest and most elegant solution of this puzzle. Unfortunately, the answer is still far from clear.