Definitely. Sabrina the Teenage Witch started as a comic and later expanded into different entertainment mediums. The comic introduced the beloved character and her magical adventures to the world.
One possible fanfiction idea could be that Sabrina gets hurt while trying to protect her friends from a dark magic spell gone wrong. Maybe she takes on too much of the spell's backlash and ends up with some magical burns or a weakened state. Her friends then have to figure out how to heal her and reverse the spell's effects.
Definitely not. Sabrina the Teenage Witch has no connection to Marvel Comics. It has its own independent comic book origin and publication history outside of the Marvel universe.
A fanfiction could focus on Salem's backstory. What if he was once a great wizard in his own right but was cursed to be a cat? Sabrina could discover this through a magical accident and try to find a way to break the curse. This could lead to adventures in different magical realms as they search for the solution.