Depending on the way the story goes and whether Octavia really does decide that becoming a pop singer is the right thing to do, a number of different arguments could be made.

If becoming a pop singer ends up harming her and leads her to make bad choices that compromise her beliefs and morals, Erecura would be the (stealth) antagonist, because she's trying to lead Octavia down a path that isn't good for her.

If becoming a pop singer is the driving force of the story and it's Octavia's true goal (and something that doesn't make her discard her faith and convictions along the way), then Danicea would be an antagonist, because she has the ability to enable Octavia's fame but chooses not to. Even though Danicea's motivations are just and in accordance with her beliefs, the only definition necessary for an antagonist is that she is in opposition to the protagonist's goals.

I'm getting the impression that Greg is kind of incidental, either way. An obstacle but not an antagonist. Then again, I could be wrong.

I'm kinda liking the idea of Erecura as Stealth Antagonist, actually!