Two possibilities-
1- He isn't actually dead yet, or rather the case of someone who died and came back to life (likely not for long) only to find himself locked inside a coffin trying to claw his way out. Horrific as that is, there have been quite a number of cases of this happening in real life. The original Fall of the House of Usher story has a very similar scene to HM's coffin scene, and I believe the ride took elements from that story (Paris' was even narrated in English by Vincent Price, who played in the 1960 adaptation of this story, and it has even more explicit homages to that story).
2- He is dead, but unlike the other ghosts who no longer have a tangible body anymore and just wander as invisible spirits, this one still has a mostly intact physical body present. So you could say he's more of a zombie situation of sorts rather than merely an intangible ghost. We know ghosts do inhabit the mansion from even the start of the ride, and they can interact with the physical world and move objects around. So manipulating a corpse should also be something they could do.
Whichever reason is the case, the coffin guy is far more explainable than Hattie. Who I believe is supposed to be a purely non-physical spirit.