OK Nitpick time.
1. Convenient, isn't it?
2. Because he's nuts.
3. Maybe he was aiming for the head and missed.
4. Because he's nuts.
5. (reason for booby traps) Because he's nuts.
6. Because he's-- oh, right.
I'm not sure all zombies know how to turn doorknobs. Morgan's wife was doing so because they appear to retain a little association with some things, and that was her home.