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.
No. Ryomen Sukuna is a fictional character from Japanese mythology and modern manga/anime. He doesn't have a basis in a true historical event or person.
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.
Fan Xian's strength was around the eighth rank. Although he had won the battle with Cheng Jushu easily, he had not surpassed the level of the Eighth Stage. In the battle before Fan Xian and Teng Zijing joined forces to deal with Cheng Jushu, his strength had increased. He also said that after that battle, his zhenqi had gone up another level. Fan Xian's hearing had not reached the level of the ninth-level ace Yan Xiaoyi, so he could deduce that his strength should be between the eighth and ninth-level. However, the specific level of the expert was not explicitly mentioned.
Fan Xian's strength was between the eighth and ninth ranks. Some documents believed that his strength had reached the Eighth Stage in Northern Qi, but he was no match for the ninth-level experts Yan Xiaoyi and Friar He. In addition, Fan Xian had won easily against Cheng Jushu, but he had not surpassed the eighth rank. However, after this battle, Fan Xian's strength had increased. Thus, it could be inferred that Fan Xian's strength should be between the eighth and ninth ranks.