I don't believe Holt is a mastermind. I really think he is someone else's puppet. Just don't know who's yet. The only mystery I'm chasing down is the draining bathtub water. It happened to Vivienne early on and it happened to Abeer - the bathtub water keeps draining. I don't know why but it could be connected to a weird gurgling sound Abeer heard on the beach. Like somehow the water in the baths drains out into the ocean. Maybe Holt is so rich that he had an entire fake ocean made, and it's all just re-used and recycled for baths and showers. That's why there are 'icebergs' in the tropics. The entire island and the ocean around it is just a set!
Another theory has always lurking in the back of my head that it's something else that I don't really wanna put that out into the universe... but I think we all know who's hosting. Jesus take the wheel but the dark days could be upon us again... perhaps opening that door was a bigger deal than we first thought.
What if the seven pendants unleashed the dark days on the world?