I want to discuss a factor that hasn't been added yet: fuedualism.
After the fall of Rome, power was taken and split up among powerful waring princes, the church became very powerful, and life for the people became a game of "who can please most well the powerful?"
Which is, to my mind, what would precipitate out from a zombie crisis: a massive return to religion, power further concentrating around existing machinations of authority, such as police and military.
Almost everyone would decide that it is better to be a slave to a powerful group of people than to suffer outright the slings of fortune alone. Only about 25% of americans own a gun, and most of the people who do don't know how to clean it and furthermore keep fewer than 50 rounds on hand at any given time. Anyone capable of surviving must also be capable of administrating over and protecting the nonsurvivors who will spring up and cling like vines.
Liked in medieval times, a wise prince would find occupation for them: growing food, collecting food, erecting infrastructure, creating walls and barricades. Occupation is very important, both in terms of the labor completed, and in terms of control. A busy pot never boils.