It is indeed a true story. The events and characters in the film draw inspiration from actual happenings in Roxanne Shante's life. However, some artistic liberties might have been taken for cinematic purposes.
One key element is her youth. Starting so young in the rap scene was a big part of her story. Another is the competition, especially the Roxanne Wars which put her on the map.
Roxanne would likely be terrified at first. She's in the hands of a known villain. But being a strong - willed journalist, she would quickly start to look for ways to escape or at least understand Megamind's motives. She might try to talk him out of whatever evil plan he has in mind, using her skills of persuasion.
It depends. Sometimes stories are based on real events but with fictional elements added. Without specific details about 'Roxanne', it's hard to say for sure.