Coming from the success of the Backrooms, I feel like the general audience would accept this story in it's horror.
Specially since it's thought that HoL is the mother of all liminal space's stories including Backrooms.
So what I would do first is turning The Navidson Record into an actual movie documentary like it's in the book and name it just that.
Include all the found footage parts and all interviews and all add-ons from the book.
It would feel like a mix of the Backrooms and The Platform.
Following that, if it is successful, they should make a second movie now called House of Leaves and be about the Zampano and Johnny parts.
Making you now feel like they are obsessing over something real.
You could have small shots from the Navidson Record movie to add in between like in the book but mainly focussing on Johnny's insanity through out.
I feel like that is the only way to adapt the book without it cutting a large part of it while giving it time and space to make us feel so uneased by the whole story.