There isn't much of a mystery. Tom Straw is the real author. The connection between the fictional Castle writing the novels in the show and Straw writing them in real life is pretty straightforward. It's just part of how the show's universe is created with this fictional - in - show author and the real - world author behind it all.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you might be able to find some really good ones about Rick Castle and Natalie Rhodes there.
One great story could be 'A New Adventure' where they team up to solve a mystery that involves both of their pasts. It has a lot of character development and interesting plot twists.
Rick's origin story has some key aspects. He was born into a world with certain scientific limitations, but his mind was far beyond those. He started breaking boundaries early on. His origin might also involve some sort of trauma that made him so detached from normal emotions. We see his character constantly using alcohol and going on wild adventures as a way to cope or perhaps forget his past.
Well, in the world of fanfiction, people can take characters in all sorts of directions. But in the context of 'Rick and Morty', the idea of Rick abusing Morty in the fanfiction is a deviation from the show. The show focuses on their interdependent relationship, with Rick being the brilliant but flawed mentor and Morty being the somewhat naive sidekick. Fanfiction writers might create such scenarios to explore the darker side of their relationship, but it's not a part of the main narrative.
There are indeed Rick and Morty comics. They're a great way for fans to immerse themselves deeper into the world of Rick and Morty. The comics sometimes explore concepts and characters that might not have been featured prominently in the show.