The NMO mount itself has an O ring under the outer nut. If properly installed at the shop, it'll keep the mount watertight and keep moisture out of your vehicle.
The outer seals are there to keep water out of the antenna base/NMO connection. You do want the correct seal installed. As stated above, they need to stay with the antenna.
Not sure exactly what you are referring to.The seal on one mount the 2m is under the gold female screw thingy. I don't think it will come off unless I unscrew it. Do u know how it cones off? Or should I just switch it back?