Those are all terrible reasons for why Prowler shouldn't be used.
1) Yes, Prowler's a hero now, but he started off a villain. Ronan had the same situation, yet they were able to use him as the villain for GotG.
2) You could say the same thing about characters like Rocket Raccoon, Nebula or the Inhumans, yet Marvel is clearly using those types of characters in their movies.
3) Being C-list or below doesn't matter. Comicbook fans are the only ones who still think like that. To the general public, all of these characters are essentially unknown. And that logic doesn't really work because if you look at something like Star Wars, did anyone know who Luke Skywalker was before the first movie came out? Did anyone know who Martin Riggs was before the first Lethal Weapon movie? And really, I don't think Kevin Fiege gives a **** whether certain characters are Z-list or not, as long as they have potential to be used in new and interesting ways. As mentioned before: Rocket Raccoon, Groot, Nebula, the Inhumans, Jessica Jones, etc.