Revealing mistake: When the Lawyer plays the videoclip on the laptop, the stop button is pressed down all the time, instead of the play button. (00:14:40)
Revealing mistake: When Arthur hits Cyrus at the end, Cyrus flies back and hits the floor. The floor is meant to be solid and sturdy, yet clealy wobbles when Cyrus hits. (01:18:35)
Suggested correction: Cyrus was an occultist, it is presumed that this video was haunted. That can also explain why he answers what they key is for in the video after someone asks what it's for.
This theory does not hold up because Cyrus is alive. He's just been wearing a prosthetic neck piece with a popped blood pack in it to appear as a ghost the whole time, in case he's seen walking the house during the plan.