I binged The Watcher last night, after sleeping on it came up with my own Theory. My theory is the entire town is in on it, just on varying levels which ultimately ties back to the main theory of human greed and keeping things traditional vs. resistance change.

Detective Rourke Chamberland - Single. Drives a super nice car and has ED. Owns the entire police department and has influence with the FBI for years. Isn't available the day the cops come when the police are called to the house. Sits on the DNA test. Withholds additional police support when the family asks for it. Isn't helpful in the slightest. "We'll send additional patrol car's." When Theodora Birch asks for the files on the house, he gives a funny reaction and says "sure". And a dusted file box only leading up to 1995 is available that's been ripped open already. Doesn't help send support for the family or Karen when they need it.

Theodora Birch - Dying of cancer and needs money. Willing to do whatever (takes a pay cut to pick up the case). Her motive is to make money and cover cancer treatment and she's helping guide the family in her theories. She is driven by money, not by interest of the family.

Karen Calhoun - Super pushy on sales. Pushing the family to divorce and sell the house. Always looking for a sale and all these shady LLCs. I think she was just a greedy real estate agent that plays into the theme of The Watcher. How human greed takes over. She never even got the chance to install security.

Dakota - Local security that started his own "Business." Puts his security card after the new family moves in after he knows what the previous family (and Karen) just went through investing in security systems and cameras. As a security person, don't you think you'd let the family know what just happened in the house?! No, let me just solicit more business for the next poor family sitting right outside to prey on them. Don't even get me started hooking up with the guys daughter and putting a hidden camera in the bedroom and using that to blackmail the guy. Let's just brush that off. And his username being "The Watcher" and him claiming it's just a coincidence

John Graff - After finding the files about the "John Graff" family. He stops by to make himself a sandwich and meets Dean as they are renovating the basement. When they find the tunnel in the basement where his bedroom is and he's running to Pearl's house and she say's "They're onto us". Clearly Pearl and John are involved. They gave it away right then who the main culprits were. I am surprised they showed who was running and which house he ran to, I figured that would have been the mystery.

Mo / Mitch - At a minimum would relay information to Pearl. The murder of two people at the house was just embarrassing part of the show and would be national news if what they mentioned was actually true. Family suicide turned out to be two homeless people for insurance claim. People just moved on and brushed it under the rug.

Pearl / Jasper - Definitely were the main antagonist. They would break into the house (deliver the newspaper, break into the elevator thing) trespass on property, try to keep the trees. Always trespassing. And the John Graff scene when he runs into their home and is included in those meetings.

Jasper - Could talk and helped with leading the meetings. Either him or John Gaff literally committed huge crimes killing a ferrett or Karen's dog. It's one thing you break and enter, scare and freak people out, but that is literally murdering animals and a next level of psycho path.

Roger Kaplan - At least was the origination of the letters Ode to the house and went to visit the house when it was for sale (as well as John). So clearly some connection between them.

Dean - Sold all his assets to buy the house, which leads into the theme of "greed" or overextending themselves, which eventually leads to him writing a letter himself.

TLDR: With all the shady things going on in the town, there's noway multiple people weren't involved just on varying degrees driven by greed and desire. The letters were focused on greed. Corrupt cops / security, sleazy sales people, obsessed people / traditionalist.