I'm not sure who the author of the 'Robbie short story' is. There are so many short stories out there and without more information, it's impossible to determine the author.
It's a bit hard to pin down an exact age. Different storylines and timelines might give varying indications. You could check with comic experts or in-depth wikis for a more accurate answer.
Another fanfiction could be set in an alternate universe where all the characters from Victorious have superpowers. Ariana Grande's character discovers she has the power to control sound, which she uses both for good, like helping her friends in their performances, and for some comical mishaps when she can't control it properly.
Some popular Beck and Robbie fanfics might include those that explore their friendship in different scenarios. For example, there could be fanfics where they go on a road trip together and face various challenges along the way. These fanfics often show their unique personalities and how they interact with each other.
Writing sexual fanfiction about a real person like Margot Robbie is unethical. People have a right to their privacy and dignity. Fanfiction should generally focus on fictional scenarios and the creative expansion of fictional characters' stories, not invade the personal boundaries of real individuals through inappropriate sexual content.
The character played by Margot Robbie in Bombshell is indeed fictional. It was created specifically for the movie to add depth and variety to the story.