People have covered most of your points, but I will say something about the survivors' accuracy. Lizzie is a naturally good shot, some people just are. They pick up a rifle, get the training and are bullseyeing targets. I have heard of more women where this happens. As for the rest, they aren't always shooting the head. KC said during the live Season 2 finale that the zombies will die from torso damage as well. Like when Burt shot the Jumper in the chest. The survivors aren't always getting headshots, they are getting body hits too, larger target to hit.

Also for Burt's amazing accuracy, there are some people who can do such feats. Annie Oakley is one the most famous trick shot artists, Carlos Hathlock was an amazing sniper in Vietnam, Rob Furlong (shout out to fellow Canadian) can shoot amazing long distance with both left and right hands. Also go on Youtube and watch FPSRussia, your eyes will thank you. That guy once shot a balloon behind him with a pistol while only lining the shot up with a hand mirror.