Yes, shorter shells are no problem.
It should be marked on the barrel or recieiver.
Can you....I think you mean Can you 'SAFELY' leave shotgun shells in a hot car? The short answer is yes, but it will degrade the quality of the ammo.
Yes
410 shells only and only what the barrel is marked for.
The shell size is normally stamped on the barrel near the reciever. Shells have been made in many sizes over the years ranging from inch & a half mini shells up to three & a half inch magnums. If you cant find a size marked on the shotgun you should have a gunsmith inspect the gun to determine proper size. A shotgun that old cannot fire steel shot. The barrel steel is too soft and will be ruined by steel shot.
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, but beware that many blank shells contain corrosive gun powder. Clean your shotgun after firing blanks to avoid pitting your barrel.
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.
Deer slugs are the shotgun shells to use when deer hunting.
There is no "break in" shotgun. A "break open" shotgun is a single barrel, double barrel, or combination rifle/shotgun which breaks open in the sense that a lever will unlock the action, allowing the barrel to pivot at a hinge at the front of the receiver, pivoting the rear of the barrel up to expose the chamber. A shells may be loaded into the chamber and the barrel pivoted back to close the action. The act to "break in" a shotgun means to fire it a few times to loosen it up.
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.
It will fit on the frame but the action will not function with 3 inch shells.