Is it permissible to write fictional stories about matters after death, such as the torment of the grave, Paradise, and Hell, while adhering to what is mentioned in the Quran and the Sunnah and supporting them with authentic verses and hadiths?

There is no way to know unseen matters except through revelation. There is no religious impediment to writing purposeful stories about unseen matters if the writer adheres strictly to the texts of revelation. This might even be desirable if it serves or clarifies the sound doctrines of Islam, whereas some scholars have prohibited it as a way of blocking potential pretexts [for error].