I like the idea of the patients being unknowingly exposed to drugs to blur the line of what is rational and what is not.
I think it would allow the writers to take a good crack at bending reality.
Also the mole patient who subtly exposes plot-keys would be cool.

Personally, I am a fan of mashing together as many potential plot-twists as possible right off the bat. It makes the timing of each twist much more natural.

So, let's say instead of being guests at a rehab center, the characters are simply vacationers who make reservations at a resort which has a perfectly legitimate history and no reason to suspect anything outside of it being the best 5-star vacation spot (I still picture this happening on a postcard-esque tropical island). The vacationers all just happened to pick the worst time possible to book their rooms.
What if the experiment to come is the first time anything like it has happened there? Maybe a private group of researchers decide to buy out the place.
They plant a mole in the group (as you suggested). I guess the mole would have to be somebody very well experienced with espionage, if the private research group has the money to buy out an entire island for a few days. Maybe an intelligence agent who just wants to take a vacation, mess with some people's heads and make some cash on the side...
There could accidentally be a genuine psychopath in the group, maybe something similar to the character Dexter...
Possibly a wannabe paranormal investigator, or at least a devoted believer. However, I would probably want that particular character to be demon-fodder.

I think that would eliminate the possibility of the characters brushing off potentially shocking scenes. It would also generate an inherently unstable character dynamic, justifying things like small cliques developing or (warning, cliche) individual characters ditching the group.

Another point I was considering is that most paranormal stories have absolutely terrible endings. I mean, in most movies the survivor(s) remain wimpy, shivering, useless asshats. I would like for these characters to eventually accept the inevitability of their own deaths and go to extremes to survive. Kind of like Evil Dead.
Everyone likes Ash, everyone likes Harry Mason, everyone likes Chris Redfield. The reason why is because they eventually grabbed up their guts, picked up that hatchet and did the damn thing.