Not the same actor.
Reenactment Mason was played by Charles Malik Whitfield
Joe Alvarez played Ghost Corpse Mason
Dude, its Beacon Hills.

He died before the blood moon, so if he was killed he wasn't killed by ghosts. It could have been a corpse with some of Mason planted on it. He was burnt to a crisp, they could have just yanked some of Mason's teeth out and he could still be alive and be in on it. They never really said how they confirmed it was Mason's body.

I think it's actually supped to be the actor who was hired by Sidney. Correct me if I'm wrong but did Mason die during the blood moon? Because if not then he can't come back as a ghost.

It was him, but it doesn't make sense - that was the actor who played him during the re-enactment, who presumably is not dead

Yes.
Dude, its Beacon Hills.