Brain is an anomaly rather than a traitor. He is actively working against MoM's design based on one of the Secret Reports. A traitor would be someone who tries to betray the others and work against them in secret, but Brain is playing with open cards. I'd imagine him being Ava's doing actually. I can see Ava deciding not to add someone, who MoM put on the list, but instead add Brain.
Now the question is, who killed Strelitzia. I think it is the person, who was on the list, but was swapped out for Brain. This person needed to target someone easily killable, that's why it was specifically Strelitzia. With her out of the picture, the fifth could easily infiltrate the Dandelions (as he was originally supposed to do so) and ensure that the MoM's design comes to pass.
Based on this, I too agree that Ventus is still a probable suspect.
- Ephemer and Skuld are known for their connection to Ava, and shares scenes with her.
- Brain is playing with open cards at this point, as he revealed his agenda.
- Lauriam is her brother, and based on the dialogue with the nameless star in KH3 I think we can deduce that it wasn't him.
Ven is the odd one out. Not only because of Vanitas' existence (which is unique in a series actually), but also because he's very different from the Ven we know in our timeline.