Okay, let's get started.
Letters EP3
The two letters in EP3 aren't from the Nerds. They're from Tybalt. Why? Chad only attacked the MC because he thought the MC would send him the pictures. Chad only thought that because the MC spied on him in EP2. Anyone who looks closely at that scene will see a tennis racket and self-tanner on a bench in the locker room. Both are items Tybalt uses. Later in episode 7, Tybalt tells the protagonist that he did not play with his own tennis racket. Why not, you ask? Chad smashed it in a fit of rage in EP2. Tybalt caught Chad, his sister's ex-boyfriend, in the locker room with Troy, took a picture, and later sent it to Chad.
There's only one question left to answer: How did Tybalt get the pictures from the DIK laptop? That's a tough one, but I suspect Nick is behind it. It's a faint clue that leads me to suspect this. When Maya and the MC are on their rounds in Episode 1, we see Tybalt on the phone outside the Mansion. Then, when they arrive at the DIKs' place, we see Nick on the phone outside the Mansion.
I'm thinking that Tybalt wanted to get revenge on the DIKs because they were in his room again, and he saw an opportunity to get rid of a rival (the MC).
Cathy's pictures: The ALPHAs didn't initiate the Cathy Cluck, but they did distribute pictures of Cathy around campus. They simply took the picture of the Cluck and copied it multiple times.
Episode 10
We already know who distributed the pictures in Episode 10.
Letters in Episode 11
Plan A is designed to ensure that neither of the two main instigators nor any accomplices are caught.
Plan B is designed to keep the two main instigators secret, but make one accomplice a martyr.
The main instigators are Isabella and Sage.
The accomplices are the NERDs, with Magnar becoming the martyr in Plan B.
Sage and Isabella have been working together on Sage's dissertation since Episode 7. In Episode 7, Isabella already told the MC in the car that Tybalt would get his just punishment and that she found the Mobbing Clucks (Sage Clucks) quite funny. Isabella is also the ruler of the NERD realm, where Sage has been several times due to her dissertation, as we learn in Episode 9. Sage tells Isabella that she already knows her keywords and has seen Isabella in this outfit. Sage is also practicing calligraphy, as you should have seen in Episode 8. Nothing was planned until Episode 9. But since Episode 9 and in Episode 10, all the LIs become friends. Then the whole thing with Tybalt's melon sex happens. Jill and my MC agreed that Tybalt shouldn't be deprived of everything, including his presidency. Sage and Isabella hate Tybalt, and I think that's why they would have definitely decided differently. Therefore, both Isabella and Sage start working on a plan and look for allies, which they find among the NERDs.
The envelope addressed to Anthony is meant to sever the connection between the HOTs and the ALPHAs. Sage already learned in Episode 8 that there was something more between Ashley and Derek. Derek had also become a good friend of Sage's, and she would have been keeping an eye on things. Then, at the DIKs' Christmas party, Ashley and Derek got together, which is why Anthony received the envelope. The Thanksgiving barbecue was another factor in the decision, as everyone involved heard that Arieth and Anthony slept together there.
The envelope addressed to Dawe is her form of revenge against him for spreading pictures of Cathy on campus, for always talking or writing shit, and for ordering the destruction of the DIK mansion.
The envelope addressed to Chad is also a form of revenge, but at the same time, it's meant to be a liberation for Chad. That's why Caleb was told to open the envelope at the ALPHAs' Christmas party, so that it would be from Sage. Caleb was manipulated into opening the envelope in front of all the ALPHAs. Chad would then have no way to back out.
The real stroke of genius, however, is Tybalt's letter. In it, he is blackmailed into resigning as president and publicly admitting to writing the EP3 letters. This, he claims, could restore peace to campus. If he refuses, his melon sex pictures will be released.
If everything goes according to plan, then Plan A will work. No one will ever know who wrote the letters in Episode 11. But on a battlefield, Plan A rarely survives. If it were to come out that Isabella and Sage were the main instigators, it would be possible for them to lose Jill and the MC's friendship. They don't want to take that risk, hence Plan B: Magnar as the martyr. He enjoys that role anyway. He takes all the blame, thus protecting not only Isabella and Sage, but also his NERDs. This also explains why Sally isn't allowed to be around the NERDs anymore. Her proximity to the HOTs and the DIKs jeopardizes both Plan A and Plan B. Now, what's in it for the NERDs? Isabella is taking things a bit easier with the library, and Sage is a Burke, the Dean's daughter. It's always good to be able to call in favors.
But Plan B also goes awry because Maya opens a door. Maya sees a flip chart with pictures and notes. She looks at the MC and waits for his reaction. However, the MC is not horrified by the flip chart, but by a picture in which he sees Isabella, James, Neil, and his pregnant mother.