In the manga, Sukuna doesn't possess all 20 fingers. There are specific story arcs and events where his finger count changes based on the plot developments.
In the manga, Sukuna does acquire all his fingers over the course of the story. This is a significant plot development that has major implications for the storyline and character dynamics.
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 reason why Wen Zhong had three eyes was because he had cultivated for thousands of years and gradually comprehended the laws of the Heavenly Dao, thus obtaining the power of the Heavenly Eye. According to legend, Wen Zhong was born with a third eye. It had already existed when he was born. His third eye was known as the Eye of Divine Power. It could distinguish between good and evil, and help him become an important official of the Shang Dynasty. Some legends said that Wen Zhong's third eye came from Goddess Nüwa, and humans had a third divine eye to begin with. They could only open it by cultivating the Great Dao. In short, Wen Zhong's third eye was the result of his cultivation and birth. It had a special ability.
One possibility could be a magical experiment gone wrong. Maybe she was trying a new spell that had unforeseen side effects on her appearance, specifically causing her eyes to turn red.