WMG / The Haunted Mansion

The Hatbox ghost was Cousin Huit.
Disney released two souveniers featuring the Hatbox ghost with a tombstone from the queue. One says "Rest in Peace, Cousin Huit, We all know you didn't do it." Hat box ghost's head appears as part of the tombstone. he other one says " Here lies Fred; a rock fell on his head." The O-Pin house event revealed that Hatbox ghost goes by the name H.B. Ghost. Of course H.B. could very well just mean hatbox, but H. could also stand for Huit and B. stands for an unrevealed middle name. Huit was accused of killing some guy named Fred with a rock. Huit was found innocent at his trial, but Fred's family believed that Huit was truly guilty. They took the law into their own hands and beheaded Huit. His head was hidden in a nearby hatbox. Huit's family insisted that he was innocent and wrote it on his epitaph.

In the earliest days of the Mansion, the Bride was malevolent.
One of the motifs of the opening-days Attic was heads in hatboxes. There were several hatbox props with heads that would pop out of them, like the pop-up ghosts. The Bride seems to be the central figure, and her heartbeat dominates the room. After the cars passed the Bride, they were met with the Hatbox Ghost, whose head was supposed to disappear and reappear in his box. With the preceding props and the accompanying horror cliche, one was supposed to assume that Hatty was done in by the Bride, which is a concept later reused for the Constance character, and possibly even by HBG's return. The very first version of the Bride figure was made to appear as a skeletal corpse, which allows the connection to be made far more easily. However, since the Hatbox Ghost didn't work, the pop-up heads lost their buildup and the Bride had to become the climax. From then, the original Bride was known as a mournful figure. More info can be found here.

The Hatbox Ghost was a resident of Halloween Town.
He's got a similarly quirky and spooky concept as the denizens of the town, and the yearly overlap with the film makes it all the more likely.

Constance is the opposite of Rosalina.
Don't worry, there is an amount of logic here.
  • Both are women associated with the color blue.
  • Rosalina represents life and rebirth, while Constance represents death and murder.
  • Rosalina is of a high position in the universe and looks like royalty, yet is quite humble and willing to help, while Connie was a poorer woman who killed her husbands to climb up the social ladder.
  • On a similar note, their attire. Rosalina is Simple Yet Opulent, but Constance craves wealth and jewelry, proudly adding a string of pearls for each husband's death.
  • Rosalina stopped aging in her twenties or so, but Constance's ghost makes it seem that she died at around the same age.
  • Constance has had five marriages ended through murder, but Rosalina's had no semi-official relationships.
So there it is. Without the context of Constance being a ghost trapped on Earth and Rosalina being a powerful guardian of space, it could make sense.

The woman in the stretching gallery is Abigale Patecleaver...
...which is an alias for "Constance Hatchaway". Constance took a big risk by apparently keeping her name the same during all of her murderous marriages, but perhaps she changed it later in life, her twisted sense of humor deciding to keep a reference to axe murder in her last name.

The Mansion itself is trying to thwart Constance and bring her to justice.
This is based on the theory that the house itself is possessed by spirits, not only inhabited by them. Your first clue is in the stretching gallery, in which we see a portrait of an old woman hold a rose pan down to show that she's sitting on the grave of "George", who apparently took an axe to the brain. Later, in the attic, this is mirrored, as the wedding portraits show the same woman with each groom's head fading in and out, and some of the gifts showing a headless groom. The final portrait is the nail in the coffin, showing Constance holding the rose in an identical pose, with her husband being the George from the stretching painting. However, Constance herself has caught on to the ghosts' interference, and appears wielding an axe, but you pass her unharmed, only to come face-to-face with the Hatbox Ghost, who hints that the boxes in the attic contain the heads of the grooms. In other words, the ghosts of the Mansion have given you the information and the evidence of Constance's crimes, and are hoping that you would be able to tell the police to prove that Constance was indeed a Black Widow, which would presumably get her out of the Mansion and free the husbands' spirits.

The Caretaker is a ghost, too
He and his dog died of fright the first time the ghosts appeared before them. Their ghosts still stand there, still not having realized that they're dead now and no longer have anything to fear.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/TheHauntedMansion