I agree with some of your points Hugh. I do find the zombie lore a bit harder to figure out than, say, vampires or werewolves. You're right...Do They Eat? If so, what have the guys walking around the yard been eating? What happens if they don't eat? Do they die? but they're dead. Maybe those still "alive" are eating the dead zombies? Who knows?
The one thing you said at the end,though, is that while they all have the virus, (and thus will all turn when they die), cutting the leg off makes sense, in that if the poison (or whatever it is) doesn't reach the part of the brain in question, they can stave off immediate zombification.
But there are several inconsistencies, I'll grant you that.
EDIT: Just thought of soemthing. We saw plenty of dead zombies in the prison grounds. I guess they starved?