Yes and no. The yes comes from simple fact, if the projectile can move down the barrel, it can be fired. I've seen .221 Fireballs fired out of a 7TCU barrel. The no is because you cannot do it well. In effect, if you shoot a slug down a smooth bore shotgun barrel, you will have little in the way of accuracy. Slugs should be shot from a slug barrel, and shot from a smooth bore.
Yes.
Depends on what it is choked.
Yes: A Remington 870 shotgun with a modified choke will shoot any commercial shotgun slug.
Any non rifled slug. However for best perfomance you want a "saboted" slug.
Yes, I don't think you should have a problem, but your pattern may not be consistent.
Yes, but, DON'T DO IT!!!!!!!!
Yes, if it is a 10 gauge shotgun.
Yes
Do not try it, the likelihood of the gun exploding in your hands is very high.
no
The Stevens 59A can shoot all size loads from #12 to slug. So in short, yes.
As long as the owner's manual doesn't prohibit it.