Yes but not the other way around .You can not use 3" shells in a 2 3/4 chambered gun.
Yes
Yes, shorter shells are no problem.
yes.
All day long. Any shotgun can handle shorter shells than it is chambered for as long as they are the same gauge.
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.
12-70 is frequently used to indicate the gauge of the shotgun followed by the length of the chamber in millimeters. In the US, or non-metric measurements, that means a 12 gauge shotgun chambered for 2.75 inch (usually read two and three quarter inch) shells. Sometimes, you'll see 12-76 to designate a 12 gauge shotgun chambered for 3 inch shells.
When were 3inch magnum
Not if you want to continue living.
yes
Yes it does I have one.
Google a company called Polywad. I have bought shells from them and was satisfied with their product.
The barrel may be able to be fitted however the shotgun would still be limited to 2-3/4" shells. A longer stroke of the bolt is required when shooting the 3" magnum shells which requires a different receiver. Not the best idea.