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.
It could be other elements from Japanese folklore. There are many supernatural beings and concepts in Japanese mythology that might have influenced his creation.
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.
The true body of the shadow was Da Bao. Lin Dabao could use his identity as a fool to make many people lower their guard against him and complete his mission. Although the shadow's true identity was not Dabao but someone from Dongyi, in order to confuse the audience, the shadow's identity was set as Dabao at the beginning.