One way is to look for fanfiction communities that have strict rules against adult or inappropriate content. Reddit has some great sub -reddits dedicated to Marvel fanfiction where users often recommend clean and interesting stories about Jean Grey. You can also ask for recommendations in these communities.
Jean Grey does die in some story arcs of the comic books. However, comic book universes are complex, and her fate can change depending on the writer and the specific storyline.
Yes, there are many. For example, some fanfictions explore her relationship with the X - Men in a platonic and growth - oriented way. They might focus on how she learns to control her powers, her leadership within the team, and her friendships.
The number of Marvel comic heroes is constantly changing and growing. As of now, there are well over 8000 characters, but new ones are always being introduced.
An OC could be a villain who has a vendetta against Jean Grey as Marvel Girl and sees Marvel Boy as an obstacle. This would create a lot of action and conflict in the fanfic. The OC could have unique powers that make it a challenging opponent for both Jean and Marvel Boy.
It's a bit complicated. Jean Grey's fate varies depending on the comic storyline and continuity. Sometimes she's dead, but often there are plot devices that bring her back.