Yes
Yes, shorter shells are no problem.
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.
yes
the rumor is any serial number on an 870 express ending with an M means its a magnum. I have one I bought in about 1987 and have shot thousands of three inch shells through it.
Google a company called Polywad. I have bought shells from them and was satisfied with their product.
Yes but not the other way around .You can not use 3" shells in a 2 3/4 chambered gun.
Yes you can you can shoot 3 or 2 and a half in
If a shot has a 3inch chamber it will also shoot - 2 3/4 inch shells..
No. The 2 3/4 means Two and Three Quarters Inch shells (or shorter).
It will fit on the frame but the action will not function with 3 inch shells.
375-550 dollars, depending on overall condition of your shotgun.