What might happen if I received 7 traffic violations in Italy for being in an unauthorized zone while on vacation and choose not to pay them?

I am somewhat surprised that you were able to return to the United States without paying the fines for what you have written about. Essentially, there is nothing that the municipality can do to you now that you are back in this country.

I would not pay the fine but beware about returning to that area in Italy in the future if you do not pay the fine. If it is discovered that you are back, you could be arrested for the outstanding unpaid fine.