I can see why Octavia doesn't want to turn on Stella.

While she is annoyed by Stella's behavior, Stella is the only person she has in her life right now. She can't stay with Stolas and she can't even answer Stolas's calls. If she admits that Stella is a terrible person and an abuser, Via will just seep into a depression after she realizes that she's stuck with an abuser with no source of comfort. At least the illusion that Stella is a good person and mother can make Octavia's life bearable.