Giyu survives in the manga. The plot doesn't lead to his death. His character has many adventures and developments that keep him alive and active in the story.
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.
Their relationship can be developed through shared experiences. For example, if they both get injured in a battle and have to rely on each other for survival while waiting for help. This would force them to open up and communicate more, breaking down Giyu's usual stoic exterior and allowing Shinobu to see a more vulnerable side of him.