Defend the Manor - Massive game for Game Festival

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This weekend is the Great Plains Game Festival and I have been given a 4 hour time block to run a scenario. I want to keep things as simple as possible seeing as a) this is my first time running an event like this and b) most of the players will most likely be new to the game. So heres my thoughts.

Defend the Manor Massive Game Scenario

-This will be a timed event that doesnt use the Sun Tracker. If the Heroes can survive the 4hr time block they win. What we do if all Heroes die wil depend on time.
-All boards from Base and Timber Peak will be used.
-A potential of 11 Heroes in play. Survivor Chracters will be used.
-Barricades will be used as well as the Experience system. I feel these are kind of needed but shouldnt be too much for any new players
-28 zombies wil be used
-Zombies Autospawn, Extra Fight Dice, and 6 card hand (I will be the Zombie Master!!!!)
-I dont want players to feel like they have to stay so if a player ever wants to leave their character will either be taken over by someone else not playing or go into "hiding". A character in hiding will be placed on their side, all their item cards will stay on the character card, any event cards in hand are given to the player to the left, No figure or cards may interact with a character in hiding. When another player takes control of a character in hiding they will get a free movement roll if any Zombies are with in 1 space and may move through Zombies without fighting.
-Hero Deck depeletion will not be a loss but should it be replinished or not considering all pick up abilities will be in play.
-Should I keep the Zombie Win of 9 or more Zombies in the Manor or alter this some how?
-Heroes will get their free Hero card.

So any thoughts on this? I want this to be difficult for the Heroes but still mostly in their favor. I feel like things should be fairly balanced but we will see once theres 11 hereos out at one time. If I feel like things are getting too easy for the heroes I want some things in mind that I can do to change that. For instance I may out of no where tell them Zombies now move an extra space each turn or I may spawn a couple Zombie Hereos from any Townsfolk event thats in the discard.