In short, a teenager gets duped into becoming an anomalous humanoid by a well-intentioned, but inept, benefactor. The teen's writing hasn't been precisely affected, he's been given a powerful and anomalous empathy that can affect other people and be projected into what he does.

In regards to the effects, it's not that 540 gets angry at the meanies saying bad things about his work and gets back at them, through his empathy he's able to make others feel the fundamental rejection he feels on a physical level. This is only somewhat resisted by emotionally distant/indifferent people.

As for the final message, Sebald is very deliberately using sebald code. This isn't him trying to cover his tracks or making it difficult for others to see the message, the implication here is that he imbues his messages with memetic or information hazards to try and compell people into doing what he asks. Seeing how there's an anti-memetic nature to his appearance as described by 540 in the transcript, and also the way 540 was affected by Sebald's demonstration piece, it's sort of clear that there's something off about his communication as well. Sebald might just be a pseudonym for all everybody knows.