Not recommended.
Yes
Your question needs some work- as written, has no meaning. A shotgun marked 3 inch can shoot 2 3/4 inch shells, or 3 inch. A gun marked 2 3/4 inch can only shoot those- no 3 inch shells.
You should be OK with standard loads.
If Vulcan steel barrel means very old or "Damascus" type twist steel barrels, the answer is an emphatic "NO". You're too young to die, no matter what your current age.
NO! This is EXTREMELY dangerous!
Yes, shorter shells are no problem.
You could eaither shoot dove with a 20 gauge shotgun or a 12 gauge shotgun. You should also use a bird shot shells
yes
No, you can not shoot anything but 12 gauge shotgun shells through a 12 gauge shotgun. Never use any ammunition other than what the firearm was designed for.
Not recommended.
The length of the chamber in a 2 3/4 inch chambered shotgun is 2 3/4". The longer 3" shotgun shells will not fit in the chamber, being 1/4" too long. You can shoot 2 3/4" shotgun shells, of the correct gauge, in a 3" chambered shotgun.
Yes
http://www.wikihow.com/Shoot-a-Shotgun
Any modern ammo should be fine. Your barrel should indicate whether you can shoot 2 3/4 or 3" shells. You may want to consult a gunsmith to see if you are able to shoot steel shot through your barrel or not.
A 12 gauge shotgun cannot fire 20 gauge shells.
A 41 magnum caliber revolver will shoot 410 gauge shotgun shells but is not recommended. The shot will lead the barrel rifling. There are also adaptors you can put in a shotgun that allow you to fire pistol/rifle ammunition.