I'm all in for zombie and survival games! and this one... seems hard to survive to.
I like the exert mechanic but I feel there's something missing, like not enough stuff going on (or too few options for the players). You have 4 shots per round and have to choose whether to exert or shoot at 6+ on your first turn. The odds seem to really favour the zombies ^^' Seems like the optimal strategy is to make the boss show as soon as you are in a good position and shoot it ignoring the other zombies if you can.
If that's true, the rest is tactics management, since they move faster than you, that really is complicated, even frustrating maybe.
If enemies cannot climb stuff you could just barricade on rooftops and shoot them... but you get penalty for shooting further than 6"... Dunno.
In conclusion: You really managed to build the frustrating and panic experience of being hoarded by zombies so good job on that, but I'm not sure is a sensation that many people will go for.
Formatting stuff: Maybe it's personal but I had to read too far away to know the winning condition.