The fastener on my Bandera is a 3/8" axle cap nut. I've removed mine only twice in nearly five years. Twisting and pulling the cap nut with a vise-grip pops it off fairly easily. Does one need the axle cap nut? Absolutely not.
The only time the exhaust damper should be closed is when the cooker is not in use. In my opinion, if the cooker is fired up, the exhaust should never be anything less than wide open. Never. Otherwise, it is said, creosote can build up on the inside. Yech!
If you are going to store the Bandera outside, I suggest keeping it covered while not in use. After four seasons of lackadaisical upkeep (and then a grease fire), my 'dera received a thorough sandblasting and repainting. It now lives in the garage, covered, when not in use.