The novel you mentioned might be the book, What to Do If You're Taken by the Missy. The male protagonist, Fang Ye, accidentally became attached to the Missy, Han Niannian, because he was kidnapped. The girl who was later adopted by the assistant should be Han Niannian. I hope you like my recommendation. Muah ~
It depends on how closely the movie followed the manga. But a good place to start might be the next major story arc or the chapter that directly follows the movie's ending. Check online manga guides for specific details.
You can continue the JJK manga from the chapter where the anime ended. Usually, you can find this information on manga-related forums or fan communities.
In many Geto and Gojo fanfictions, the theme of trust is crucial. Their trust in each other can be tested in various ways. Another common theme is the exploration of their past. What made them who they are? How did their past experiences shape their relationship? And the theme of growth, both as individuals and in their relationship with each other, is also often seen.