Definitely not. Genya remains outside the ranks of Hashira throughout the manga. This is because of a combination of factors such as his skills, experiences, and the overall direction of the story.
Well, the circumstances of Genya's death in the manga are quite complex. It involves a series of events and confrontations with powerful enemies. But I can't recall all the details at the moment.
David was this young lad who was taking care of his father's sheep. He was not from a military background but was chosen by God. In the story against Goliath, he showed remarkable bravery. He trusted in God's power and his own skills, like his ability with a slingshot, which ultimately led to his victory over the giant.
I'm not sure who David Fuller is in this story. There are so many people named David Fuller. He might be a character who had an important role in a particular event or a person with a unique personality in a family or social group. But without more context, it's difficult to be precise.
There's not enough given to know precisely who John David is. He could be a character in a family legend. For example, in a big family, there might be stories passed down about a great - uncle John David who had some kind of secret that affected the whole family's history. Or he could be a new character in a story being created, and the 'secret' part of the title is just to pique the reader's interest while his identity is slowly revealed throughout the story.