Keith will likely be defiant. He has a strong sense of justice and will not be easily broken by Zarkon. He might try to fight back immediately if he has the chance.
One of the main characters could be Shiro. Shiro has always been like a big brother to Keith. So when Keith turns into a girl, Shiro would be there to help him adjust. He'd make sure Keith is safe and comfortable in his new form. Also, Hunk could be a main character. Hunk would probably be the one to offer emotional support and cook some comfort food for Keith during this tough time of adjustment.
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.
I'm not sure if there is a specific 'pregnant Keith fanfic' available. It depends on the fandom and the amount of fan - created content out there. You could try searching on popular fanfic websites like Archive of Our Own or FanFiction.net.
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.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
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.
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.