One key element could be his origin as a human. His human experiences surely had an impact on his demon self. Another might be any significant relationships he had before becoming a demon.
Upper Moon 4's back story could involve a tragic event in his human existence. Maybe he was betrayed by someone he trusted deeply. This betrayal could have been so severe that it pushed him over the edge, and when he was turned into a demon, his new nature was shaped by that past hurt and anger. His actions as an upper - moon demon are likely influenced by the unresolved emotions from his human days.
If Bella gets pregnant in the New Moon fanfiction, Edward's reaction would be intense. He loves Bella deeply, and this situation would be both a joy and a terrifying ordeal for him. He'd be researching every possible aspect of a human - vampire pregnancy. Jacob would be in a state of disbelief. His relationship with Bella has been complicated, and this would add another layer of complexity. He might try to convince Bella to leave the Cullens. The Cullen family would be focused on the medical and safety aspects. Esme would be motherly and nurturing towards Bella, while Rosalie might have mixed feelings, given her own past experiences with wanting a child. Charlie would be a mess of emotions, from confusion to anger to deep concern for his daughter.
Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
It's hard to give an exact number. Different story arcs and battles led to the deaths of multiple hashira. Some died heroically, while others met their fate in unexpected ways. The count changes as the manga progresses.
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.