Activated artifacts need to reactivate every round. (Thus the "on activation" wording on some artifacts). So:
1.) Breastplate is placed "on activation." Thus, it needs to be activated every round, using up your art slot.
2.) Yes. It becomes a placed card, not a curse.
3.) Magic Charms is entirely different because it has an identifier (which is "always active"). However, you may activate and deactivate it during the normal art de/active times.