After they rip all the wall coverings down ... next thing you know Maddie knows everything about the demons (they are safe in the spell room, she can read spells, she knows what they want, etc). That was a too much to get off a bunch of junk off the wall.
If you paid attention to what Suzanne said, the story and instructions where written on the walls. By luck, she just so happened to see where the story began. I'm pretty sure they were in the spell room for about an hour or so, especially since Colin and Jason were taking naps waiting for morning to come. She probably studied more of the spells and stuff during that time. It's really up to the audience to fill in what they didn't show/left out. But, this theory would be the more logical one, if you ask me. Eventually later on is when Angela and the demons get bored and scare/trick them out of the room. Maddie had plenty of time to read what was written on the walls, being as the room wasn't that big anyways.
Now that was far-fetched, the whole shotgun/rusted nails thing. But I can see where she knew some stuff about the spell room. for all we know she could be a fast learner? it's a movie, and horror one at that. nothing really makes sense, and its supposed to be fun. but I can tell where the writing may have gotten lazy in the script.
Seriously, at the end of the movie all I could think was "Is she Buffy?"
I don't remember seeing any actually written english in story format on the walls, but then she starts telling them what's going on, the the way she walked off at the end made it seem like she "Knew along" how it would play out.
"Leave a little room for the Holy Ghost." - Sister Gloria
she was also readign the spellbook before the demons got losoe in one scene. it was never told how long was she doing that, for all we know she could have seen those spells in the book.
---------- "Common sense is not so common." - Voltaire
Plus, they also talked about the story that happened there when the original people who owned the mansion there that no one lived in for a very long time. They story is shown and was told on what happened. That is where I figured out what was going on while watching the movie.
I have a theory. This movie is kind of like the Evil Dead 2 to Freddy Vs Jason's Evil Dead 1, so maybe she's actually Lori Campbell under an assumed name.
Did anyone noticed when the demons were sticking their arms through the wall, it seemed like it was about ten of them out there, when there was only about 4 of them possessed? lol
Well another clue was that none of the demons even tried to attack them while in the room. They might have tanted or teased them but eventually they figured out that they were safe in the room. That is what I caught from watching the movie.