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.
Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
Naruto and Neji's friendship in fanfiction often delves into their past traumas. Naruto understands Neji's pain of being in a rigid clan structure, and Neji realizes Naruto's loneliness from being an outcast as a child. This mutual understanding cements their friendship. They support each other in battles, with Naruto relying on Neji's strategic mind and Neji depending on Naruto's raw power and determination.
Rock Lee might be an interesting choice. He is Neji's friend and has a strong sense of loyalty. He would be very dedicated to taking care of the baby. His physical strength could also be useful in case of any danger. And his positive attitude would be great for the baby's environment.
As of now, Deku is very much alive in the manga. The plot hasn't reached a point where he meets such a fate. Usually, main characters like Deku have significant story arcs and aren't easily killed off.
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.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.