Question: What happened to the carriage on the rollercoaster that flew off the rails?
Question: Eddie reveals that he's not even a direct descendant of the original staff but was actually adopted. Why would the Darkness still try to kill Eddie even though he had no ties to the staff or institute?
Answer: The Darkness thought he was lying to save himself, everyone killed had the seven deadly sins. Greed, Lust, Envy, Sloth, Gluttony, Wrath and Pride.
Question: Who exactly is the person that Melissa sees behind her before it attacks her?
Answer: The ghost that kills Melissa is the patient Vannacutt was dissecting. When Melissa initially sees them through her camera, what she's really seeing is the ghosts re-living their last moments.
Answer: Just a generic ghost/monster as it has no face.
Question: Watson gets absorbed into the Darkness, becoming a part of it, so how was he able to appear and open the attic window to let Sara and Eddie escape?
Answer: The Darkness was so focused on Sara and Eddie that Watson used his few seconds of freedom to help them.
Answer: Its just an illusion by Price. His rides are meant to confuse you. Like the tower ride looked like the cable broke and was falling to the ground.