First, find like-minded collaborators with similar writing styles and interests. Then, clearly define the story's theme and plot together. Good communication is key throughout the process.
Doing a collab story involves a lot of teamwork. You both need to respect each other's ideas and be willing to compromise. Outline the story structure beforehand and take turns writing different parts, while constantly reviewing and editing each other's work.
Well, maybe the collab introduced ideas that clashed with the original essence of your fanfiction. For example, if the other person had a very different vision of the characters or the plot. Their ideas could have forced your story in a direction you didn't like, or they might have made changes that disrupted the flow you had carefully crafted.
You could try to salvage it on your own. Go back to your original ideas and rewrite the parts that were ruined. Cut out any elements that the collab added which you don't like.
There's no way to really tell if it's common specifically for someone named Jojo. However, in general, reading fanfiction is a popular activity among fans of many different franchises. The Jojo series has a large and dedicated fan base, so it's likely that there are fans named Jojo who partake in reading fanfiction, but we can't say for sure how common it is without more data.
Perhaps this Jojo is interested in exploring different interpretations and stories within the Jojo universe that fans have come up with. Fanfiction often takes the existing characters, settings, etc. from the original Jojo series and creates new and unique storylines. So, this Jojo might be a big fan of the series and wants to see what other fans have imagined.
It depends. Maybe there are some telltale signs or elements that suggest it's a JoJo reference fanfic, but without specific context, it's hard to say for sure.