Batman would probably isolate himself at first. He'd retreat to the Batcave to analyze the situation. He might feel a sense of guilt, thinking he should have seen the betrayal coming. After that, he'd start planning his counter - move. He'd use his vast resources and knowledge to expose the truth behind the betrayal and take down whoever is behind it.
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.
She could be devastated. Wonder Woman might go into a period of mourning, isolating herself from others as she grapples with the loss. She was used to having Batman as an ally, and his death would leave a big hole in her life.
Some fans might be really excited about it. They love to see new and unexpected relationships explored in fanfiction. It gives a fresh take on the characters and their dynamics.
Some might be shocked. For instance, Batman is often portrayed in very different ways in fanfiction compared to his 'canon' self. Superman might raise an eyebrow at a fanfic where Batman is overly emotional. Others like the Flash might be really into it, finding the different takes on Batman's character and his adventures exciting.