Definitely not. Deku's character arc in the manga is centered around his growth and development as a hero. There are challenges and difficulties, but he never turns to the dark side.
Deku remains a hero in the manga. His determination and sense of justice keep him on the path of righteousness. There are no indications or plot developments suggesting he becomes a villain.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
Yes, there are quite a few. You can find them on platforms like Archive of Our Own. Some fanfictions might explore different aspects of a female Deku in the My Hero Academia universe, such as her relationships with other characters, her journey in becoming a hero, or how she copes with the challenges in a different way compared to the male Deku in the original story.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.