Yes
222, 223 and 9mm
No such thing. Colt did make some AR15's in electroless nickel.
You could get one if you place a custom order.
Yes
Yes
No
Did marlin make a goose gun with a thirty nine inch barrel
.380 colt stick a pencil in the barrel, jamb a potato over the pencil onto the barrel, remove the pencil
If your 20 inch barrel has screw in chokes then get a full or extra full turkey choke tube and you'll be fine. Just make sure to pattern the shot with your particular barrel and choke combination. Also make sure you know where the point of impact will be with your sights. The 24 inch barrel would be fine also as long as it's properly choked etc.
Yes
Probably.
We need make, model, condition. You gave us make. Need more information.