In this fanfiction, it might be that an enemy has found a way to manipulate the Speed Force to harm the Flash. For example, they could be using some sort of device that alters the nature of the Speed Force in the Flash's vicinity. So, the Flash has to not only deal with the pain but also figure out who is behind this and stop them before the damage becomes irreversible.
He could try to isolate himself from the Speed Force for a while. By cutting off his connection, he hopes to stop the pain, but this also means losing his speed powers temporarily. It's a risky move, but if he can figure out how to protect himself during this time, it might be a way to start healing.
Speed Demon was one of the villains of The Flash. He was a supervillain who used extreme speed and had once appeared in the story of The Flash. As for the background and abilities of the Speed Demon, further information was needed to confirm it.
In certain comic universes, the Speed Force has faced significant events that could be interpreted as its 'death'. However, comic storylines are complex and can vary greatly.
Ruby would be a likely candidate. She's already known for her speed and agility in combat. If she were to be affected by the Speed Force, it would enhance her existing abilities to a whole new level. She could move so fast that she becomes almost invisible to her enemies, and her scythe attacks would be even more deadly with the added momentum.
The top speed of The Flash in comics varies depending on the storyline and writer's imagination. But generally, it's incredibly fast, beyond what we can easily measure.
Peter Parker in the context of the Speed Force could be really interesting. The Speed Force is often associated with speedsters like The Flash. If Peter Parker were to be connected to it, perhaps he could gain super - speed in addition to his spider - powers. This could make him even more formidable in fighting crime in New York. He could use his speed to quickly web up villains or reach the scene of a crime in an instant.