Nope, Megumi is alive. The author has chosen to keep Megumi's character alive to continue developing their story and interactions within the manga's world.
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.
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.