There's some interesting stuff in here, but don't take what Baxter says as gospel.
He may be an imagineer, but he had nothing to do with the first two Mansions, and a lot of what he says about the Haunted Mansion is conflated with the work he
actually did on Phantom manor.
One thing to consider about the possible "backstory" to the original mansions (or lack thereof) is that whatever framing story Davis and Coates had in mind when they designed the thing, for all their disagreements about the attraction, one of their conclusions was that a lot of things in the ride would be better off if left unexplained. Personally, I feel like a lot of the effort in the last 20-odd years, both by fans and Disney themselves to add explanations and backstories to things like Mansion and Pirates actively detract from the rides as both artistic works (which they are) and as experiences in general. Art is sometimes defined by the word the author didn't type, or by the brushstroke the artist didn't take, and while these rides will necessarily always be subject to updates and enhancements, a lot of what's been added hasn't enhanced the rides.
As people have said, Doombuggies is a great resource for exploring the details of the Mansion, but for an exhaustive look and contemplation of the Mansion's history and possible creative antecedents I highly recommend the blog "Long Forgotten":
http://longforgottenhauntedmansion.blogspot.com/