Any spent plastic shotgun shells (hulls) should be reusable, as long as the brass is re-sized before reloading. Paper isn't recommended, and brass casings are practically non-reloadable.
I have never seen nor heard of a revolver airsoft pistol with non reusable shells. You should have no problems reusing them.
Yes.
Most commercially available shotgun shells come in boxes of 25.
Air soft shotgun shells are the size of 12 gauge shotgun shells.
Simply refill them, yes.
Find someone who reloads shotgun shells. They will relieve you of your inventory.
No
You can use nothing but 410 shells in a 410 shotgun.
NO
Shells
Shot and Slug
No.