If I hadn't made this for the "Out of Control" theme, I probably would have just gone with a click-to-shoot control scheme. I did try and make sitting and waiting a less viable strategy in general, though: that's why backing into a wall screws up your aim so badly.
I think the game runs most smoothly if you tend to stay in the centre of the city. I noticed that relying purely on pathfinding caused an enormous amount of lag once you got far away from a few zombies (I assume because they have to plot a route through the entire area to get to you) so my solution was to have them try to make a beeline initially (with the buildings laid out so as not to funnel them into too many dead ends along the way) and only start looking for a valid path once they're within a couple of thousand pixels of the player. This setup means you can't escape a horde by letting it run face-first into a wall while chasing you, but unfortunately I think it may also cause performance problems if you do manage to get a large cluster of zombies stuck somewhere. Getting close enough that they all start looking for you at once is likely to send the framerate through the floor.