At the end of episode 5, Quinn is talking to Buddy or one of his guys, letting them know she's got all the money for the weed she's been selling:
Quinn: "It's me."
Quinn: "I know I'm late. Something happened, but don't worry... I have it now. All of it."
Quinn: "No, I'm not making any excuses."
Quinn: "Tomorrow night..."
Quinn: "Bye."
We know she's late because Buddy chastises her and even cuts her take when she brings it in. And then she gets mugged and loses her cut.
In episode 6 when she's scaring Mona, she's basically just trying to threaten her not to say anything to anyone, making out that Burke would be formidable if she went up against him, but painting herself as a saviour by keeping Burke quiet. None of it's real. Burke isn't gonna say or do anything because he would be exposed, and Quinn is in the same boat. She's basically just saying a pile of shit to keep Mona quiet.
So yeah, the mc gets a diary... there's nothing linking it to any other books so far...
In episode 9 with the young Quinn flashback there is no book.
Where are you getting the idea that it's a book in the safe?
All he says is:
Rox: "You've seen Daddy's safe, right? I wanna show you something."
Rox: "And not a word to your mom, ok?"
And the scene ends. There's definitely no indication of a book in the safe.
So no coincidences so far because nothing you've said has made any sense yet.
Quinn reading the book in episode 7 actually made me think that she might have even been studying...
But there is a scene that you've missed, where the mc can actually see that book in Quinn's desk and he asks her about it:
mc: "What are you reading?"
Quinn: "None of your business."
mc: "Touchy subject?"
Quinn: "Just drop it."
mc: "Come to think of it, I don't even know what you're studying here."
Quinn: "You know. Classes."
mc: "I would never have guessed that."
mc: "What degree are you going for?"
Quinn: "Enough with the school shit, ok?"
Quinn: "You're killing my mojo here."
So her deflecting about the book is interesting... And getting the mc to suggest it's a college book is a typical DPC red herring, make a character infer something and we start to think it's true when it's not true (like when Quinn pointed out that it was the Alphas who trashed the mansion when it likely wasn't).
But the whole deal of Quinn being concerned for the mc after hearing he was threated by Vinny with a gun... that's kinda normal behaviour for anyone when they hear someone they know has been threatened with a gun...
So yeah, your whole post is flawed and smells like bullshit, but totally good to see you posting again. Hopefully episode 10 is right around the corner so we can fuel these theories with something more than manure.