Lots of "farmers shotguns" were modified over the years, even the aristocratic Model 12. A lot of them were stored in barns, which is how mine looks. But it's also an old soldier, and deserved a better retirement.
A well-made firearm is one of the few objects that can be used for centuries, as long a it gets minimal maintenance. Springs are perhaps the weakest link, but as long as someone knows how to make springs (and I think they'll be around for a long time), the gun can still function.
Ammunition is another matter. I have some original GI Krag ammo dated 1917. The National Guard still had Krags, so they were still making new ammo for it.
The ammo came in bandoliers, with ten loose rounds inside a pocket that was sewn closed with flimsy thread. I looked at some of the rounds, and each case neck was cracked, meaning air got inside the case, and oxidized the powder. If the powder ignited at all, the cracked case neck would never hold the bullet long enough for gas pressure to push it out the barrel.
I guess after 90 years that even annealed brass case necks re-harden, and then fail. Of course, drawn-brass cases were still pretty new metallurgical technology back then.
2. Would you keep ammo in a mag, or would that fatigue the spring?