Most likely not. Ghost stories often start from people's overactive imaginations or misinterpretations of normal events. For example, a strange noise at night could be just the wind but in a spooky setting like Archer Ave, it becomes a 'ghost' story.
Not likely to be purely based on a true story. Usually, such stories take some liberties and incorporate fictional elements to make it more engaging and captivating for the audience.
The story of Telegraph Ave involves its role in facilitating rapid information exchange. It was a key part of the early communication infrastructure and witnessed many technological advancements over time.
Stiles is a must - have main character. Then, depending on the direction of the fanfiction, characters like Simon Lewis could be important. Since Simon is a vampire in the Shadowhunter universe, his interactions with Stiles, who is new to this world, could be really interesting. Also, characters like Luke Garroway, the werewolf, might play a role, especially if the story involves the relationships between different Downworlder groups and Stiles' place in all of that.
The chronological order can be a great way to read the Shadowhunter novels. However, some people might find it easier to start with the most popular ones first, like 'City of Bones'. But if you want to really understand the backstory and how everything began, you could look into pre - quels first. For example, 'The Infernal Devices' series which gives a lot of background on the Shadowhunter world and is set in an earlier time period. After that, you can start with the main 'The Mortal Instruments' series in order.
I haven't read Bella's Shadowhunter fanfiction specifically, but in general, fanfictions can be really good. They allow fans to express their love for the original work in their own way. If Bella is skilled at writing, she could have added her own unique spin to the Shadowhunter universe, perhaps delving deeper into the backstories of some characters or exploring different magical aspects.