I am still confused as all hell about what Ramsley's intentions really were. It would appear as if he was trying to lift the curse on the mansion, which, if I remember correctly, was keeping all the ghosts from going on to the next life. But as we see after the curse is lifted, Ramsley's next destination was Hell. So why on Earth would he want to lift the curse? The only thing I could think of was he didn't know that he was headed to Hell, but I don't know, it still seems kinda outta whack.
As you say, Ramsley probably doesn't know what the 'next life' contains, so he won't know that he's headed straight for Hell. But as well as that, he believed he was doing the right thing by killing Elizabeth, keeping it from Gracey, and then tormenting Evers and killing Sarah. He probably wouldn't put himself down as a bad guy, even though he is transparently evil.
Ramsley was always something of The Chessmaster: he deliberately killed Elizabeth because he didn't approve of a mixed-race marriage and created an elaborate plot to pass her death off as a suicide. On that note, it's possible that the opposite of your guess is correct. He knew that he was doomed to Hell if the curse was lifted, so he deliberately tried to prevent that from happening under the guise of helping (note that he's the one who remarks that Madame Leota predicted Elizabeth's return to the mansion, suggesting that he, and not Gracey, spoke with her). Heck, Ramsley might have even known that Sara wasn't the reincarnation of Elizabeth, and invited her in an attempt to sidestep Leota's prediction by having a fake Elizabeth appear in place of the real one.
Master Gracey tells Sara his story under the claims that it's about his grandfather and he shows Sara Elizabeth's wedding dress and says she never got to wear it. So if Elizabeth died before the wedding and Grandpa Gracey killed himself out of grief, how did Sara not notice the massive Plot Hole in the story - the Gracey she's talking to shouldn't exist because his "grandfather" died without children? Even if she doesn't figure out that the Gracey she's talking to is the same one from the story, she should still realize that something doesn't add up with the family lineage.
She may have thought they had a child before Elizabeth took her own life, and that having a child out of wedlock contributed to her suicide.
Plus, Gracey doesn't say that his grandfather never married—only that his marriage to Elizabeth didn't happen. It's quite possible that Sara simply assumed Gracey's grandfather had chosen another bride (which was a relatively common practice in the nineteenth century: wealthy men pretty much had to be married).
Remember that Gracey only claimed that Elizabeth was young, not that "his grandfather" was. For all Sarah knew, the grandfather was a middle-aged widower who'd found a Second Love in Elizabeth, and who already had children.
Was Ramsley a demon of some sort? He was able to open a Hell Gate in the fireplace and summon evil spirits by saying "Damn you all to hell!" Not to mention the threat he gave the other two ghosts, saying "there are worse things than purgatory", implying that he had the power to send them to hell too.
I think he was more a ghost that surrendered to the "dark side," as it were. As noted above, he's a pretty evil guy, and by the time he starts making those threats and opens the portal to Hell, that evil side has been exposed. He may have just finally been showing his true colors by invoking evil powers.
Why didn't Ramsley just burn Elizabeth's letter instead of locking it in the attic and hoping Gracey never found it?
If Ramsley believed his racism was shared, whether openly or secretly, by the police and judges in the area, then he might have considered it potentially exculpatory evidence - an excuse for them to let him off the hook for the murder, should his crime ever be uncovered while he lived - rather than as something incriminating.
I have always asked myself since I visited the ride, but...how the Ghost Host managed to hang himself from the ceiling of the room with no doors nor windows if the ceiling is very high? There are no stairs and it's impossible that he could have jumped too high and especially with a rope to hang himself, as to do so, he would have needed time to tie the rope to the ceiling and then hang himself, which he couldn't given the lack of stairs...
In the ride, what is Constance doing? She's the only malevolent spirit in the house, but she's just hanging out in the attic showing off her axe and saying wedding vows? It's a really nice addition to the ride, but it honestly makes no sense. And why did the Imagineers decide to make one of her husbands "George" from the stretching portrait tombstone? They knew that it made no sense to have them be the same man (since Constance died young, and the portrait shows an old woman).
What makes you think that the creators of Constance intended her to die young ? As "Long-Forgotten" pointed out, it could be that her spectral form is the one she had when she committed the crime(s) that make her a ghost, not the form she had when she died.
What will they do with the Hatbox Ghost for the Haunted Mansion Holiday? Will they give him a festive hat? Turn his box into a present? Do nothing? Or deactivate him and block his area off?
I can report that Hatbox Ghost is still there during the "Nightmare Before Christmas" theme. All they did was add a small Christmas decor to his top hat, and that's it.
For the 2016 holiday setup, "Hattie" now has a bow on his hatbox. Someone must have seen that "present" idea.
Who is the Hatbox Ghost? All we knew from his original conception was that he was supposed to be a suitor of the hearbroken Bride, and his head trick would occur in time with her heartbeat. But why does his head disappear? This is only made more confusing since this is now essentially Retconned by Constance, and he's clearly not one of her axed husbands. For now, the only explanation seems to be that Hatty was an interesting visual gag (and much more so upon his return).
There are literally THOUSANDS of fanons about him. The theory displayed in the fanfics "Haunted Mansion And The Hatbox Ghost" is that he is related to the Mournful Bride Emily, was a hatter when he was alive, was beheaded upon discovering some criminal secret related to Madame Leota. In the spin-offs authorized but considered to be non-canonical Haunted Mansion comics, he is Randall Pace, a sea captain who was beheaded by a mutinying second-in-command Gracey who later used the boat for pirate activity, and used the money to build the Mansion at the end of his life, becoming Master Gracey and by the same way the Ghost Host. There is a theory that he was a suitor for Constance that discovered Constance's axe before the wedding and that she beheaded to be safe. There is yet another one that she was Constance's father or partner. Hey, there are even people who assume that he's George Hightower himself !