I haven't come across one, but you could try looking on specialized comic websites or social media groups focused on comics. Maybe someone there knows if such a comic exists.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Midoriya's friends like Uraraka and Iida would be extremely worried. They would immediately start looking for him. Uraraka might use her quirk to search in the air, while Iida would lead the ground search with his speed. Bakugo, on the other hand, would be furious. He would probably go on a solo mission to find Deku and take on Todoroki without hesitation.
One great 'Best villain Deku fanfic' is 'Villainous Deku: The Dark Ascent'. It shows Deku turning to the dark side in a very believable way. The author does an excellent job of portraying his inner turmoil and the reasons behind his transformation. Another one could be 'Deku's Villainous Reign' which has some really intense action scenes where villain Deku battles against his former friends. It's a thrilling read.
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.