There is a way to quit to menu - open the settings screen by clicking the button in the upper left corner of your screen, just below your city name. There click "Quit".
Regarding the question to amount of zombie kills on Nightmare - that depends heavily on how soon I finish the map. If I leave part of the map unclaimed, don't invent the vaccine and leave the portal open, so I can 'farm' zombie hordes for score, it can go pretty high. However, the last fast game I finished on day 149, with all 4 victory conditions met - I think I had about 4000 zombie kills.
To the comment regarding getting fort defense below 50% on Nightmare - it is entirely possible. Nightmare difficulty requires you to get a lot of people in your camp FAST, training most of them as soldiers and expanding your walls agains edges of game grid.
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