It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.
For the Naruto fandom, bashing fanfiction could potentially turn some fans away from the overall fanfiction community if they feel that their beloved character is being unfairly targeted. In the case of Rias and Issei's fandoms from High School DxD, it could create a stir as well. Some fans might engage in debates with the writers of the bashing fanfiction, trying to defend the characters. This can lead to both positive and negative interactions within the fandoms. On one hand, it can spark discussions about the characters' qualities, but on the other, it can also lead to a hostile environment if not handled properly.
The impact on the fandoms is multi - faceted. In the case of the Harry Potter fandom, it can introduce new storylines and character developments that were not possible within the original series. Fans get to see how Harry would react in a completely different universe filled with superheroes. For the Avengers fandom, it adds a touch of the magical and the complex backstories of the Harry Potter world. This can attract new fans to both fandoms. It also encourages creativity as fans try to blend the different powers, cultures, and ethics of the two universes in their own works.
Since I'm not well - versed in Yuta Tanaka's light novels specifically, it's hard to say for sure. But in general, light novels often have simple yet engaging language, vivid characters, and unique story settings. His works might also follow these trends.