Yes, he has. Markiplier often does unique and entertaining segments, and this could be one of them. He likes to interact with different types of content related to his fan base, and reading fanfics, even the not - so - good ones, is a way to do that.
His friends like Wade and Bob could be main characters. They would be the ones who first notice the change and try to help him. They might have some really funny reactions and also help him come to terms with being a girl. Maybe there could also be a mysterious character who caused the transformation in the first place and they have to find this person to reverse it.
I don't think it's a true story. Such heist scenarios are often the creation of writers and storytellers to provide thrilling and adventurous experiences. There's no evidence to suggest it's based on reality.
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.
Sure. 'A Heist with Markiplier' is an interactive story. It starts with the basic premise of planning and executing a heist. Throughout the story, there are numerous choices for the viewers to make. These choices lead to different outcomes and multiple endings. For example, you might choose a different partner for the heist, and that can completely change the flow of the story, from the obstacles you face to the final result of the heist.
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.