OH MY GOD. I REALIZED SOMETHING.
Crook is Gavin, our father ex-cellmate! The description perfectly matches: “Well you know my cellmate Gavin, he is turning out to be okay. Young guy, on the cocky side yeah, but he’s not as bad as most of the guys in here. Just a dumb kid who made some bad decisions. He seems to be trying to turn things around and he’s keeping his nose clean.”
And it might actually prove my theory even further. The second letter mentions he changed "He started out well but he’s hanging with some dangerous guys. " that he “tries to impress” and they might “talk him into doing something dangerous”.
The guys were perhaps agents of the Dozen charged to watch for the guys in prison, and instead of getting their hands dirty they used Crook as an easy way to kill our father.
Not to mention I found a hole in my theory: if he had a cellmate, how did he not react? I mean he was stabbed 12 times! However if it was his cellmate that did it, it removes that problem.
It makes perfect sense and it fit in with my previous deductions: he has a criminal background, is perhaps intimidated into killing our father, realizes his errors and try to make up for it.