Megumi is not dead in the JJK manga. The manga continues to feature him and his adventures. The story hasn't reached a point where his character is killed off.
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.
As of the latest chapters I've read, Megumi is still alive in the manga. Usually, such major character deaths are significant plot points and would be well-publicized if they occurred.
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.