No offense to the OP, but his letter was boring to read.
~
"Mikey's got a disability - open your eyes!"
The Man. The Legend. Mario Marconi.
Health & Safety Facilitator since 1965.

Good answer.
Well, it's been seven years!
My guess is that Seth has not gotten around to feeling like it.
"Please vote to preserve the unique character of Warren…" - Robert Duvall

Whenever he damn well feels like it. That's when!
Seriously though, odds are he never receved it. He probably has assistants who filter his mail and they don't think fan letters are that important. It's distressing but that's Hollywood.

You think Seth will have a version ofStan on his next album call Vishspal where he sings about receiving a series of letters that get progressively angrier because Vishspal thinks he is being ignored?

This is a joke, right? Even if the letter did end up in front of him and he thought it was a real request you don't think he'd be ROTFL as he throws the letter in the circular file?
Must be the millennial way - send wacky letters to random celebrities hoping they'll give you a job, sight and writing samples unseen.

Chances are he gets swamped with fan mail and yours would be swimming in the sea of them. It is also highly unlikely that he would get to it first because like most celebrities, he has handlers that it would go through first before it gets to him. Which usually means that it would not get to him. You can try his social media accounts, but they are most likely run by another one of his handlers and would not make it to him. I suggest checking his social media accounts for his public appearance schedule and just give him the letter directly, which is the only way to guarantee he will receive it.