Nope. Ryomen Sukuna has no basis in reality. It's a product of the creative minds of the writers and illustrators who designed a captivating fictional character for the enjoyment of the audience.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
One of the top fandoms for fanfiction is the Doctor Who fandom. With its time - traveling and space - exploring premise, the possibilities are endless. You can create new adventures for the Doctor and his companions, or introduce new alien species. The Supernatural fandom is also very popular. There are so many different mythologies and creatures in the show that you can base your fanfiction on. And then there's the Percy Jackson fandom. It combines Greek mythology with modern - day settings. You can write about new quests for demigods, or explore the relationships between different characters from different cabins at Camp Half - Blood.
No, it isn't. Fandoms vary greatly in their nature and the demographics of their fans. Some are more conservative or focused on a younger audience. In such cases, smutty fan fiction would be completely unacceptable. It can also create a divide within a fandom, with some fans being against this type of explicit content while others may be more accepting.
Yes, it can be common in some fandoms. In the Harry Potter fandom, for example, there are quite a few 'ron bashing fanfiction' pieces. Since the series has such a large and diverse fan base, different interpretations of characters are bound to happen. Some fans may focus on characters like Harry or Hermione more and see Ron as a hindrance in their relationships or adventures, so they write fanfics bashing him.
In the manga, Sukuna doesn't possess all 20 fingers. There are specific story arcs and events where his finger count changes based on the plot developments.