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.
In this fanfiction scenario, their reactions could vary greatly. Natasha Romanoff would likely be sizing up the situation immediately, trying to figure out how this new knowledge can be used to their advantage. Clint Barton might be a bit more hesitant as he's had his own experiences with Loki's mind - games. And Bruce Banner would be worried about the potential danger Loki's true form poses, especially if it has some kind of power that could set off the Hulk.
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.