A: Well, it depends whether you’re talking about a tie at the time the Zombie Boss card is played, or whether there is a current Zombie Boss, and someone increases their zombie count to make it a tie.
In the first case, nobody attains Zombie Boss status until they have a clear lead on zombies.
After that, they remain the Zombie Boss until someone actually overtakes them in number of zombies. So if Alice has four Zombies, and Bob has three when Zombie Boss is played, then Alice is the Zombie Boss. Even if Bob gets another Zombie, making them tied at four zombies each, Alice remains the Zombie Boss until Bob gets a fifth zombie.
This does create a little bit of a memory condition as to who is currently the Zombie Boss, but this is easily overcome by placing the Zombie Boss rule card in front of the current Boss.
Think of it this way: When the Zombie Boss card is played, imagine there’s a “Zombie Crown” tossed into the middle of a playing field. If two “teams” are tied, neither can attain it. As soon as one player has more zombies, they get the “crown.” When someone catches up on zombies, however, now they’re tied with one player already wearing the “crown”… and they’ll continue to wear it until another player actually gets more zombies than them to take it away.