I think not all of them are part of the Union. Maybe Roctis and other traitors didnt go well with each other. And so Roctis joined Union, while others are a part of independent organisation.
Maybe Roctis, after being defeated, will tell that he betrayed Rai to save his life. He was ready to be called a traitor as long as he can save Rai. He will say he betrayed other nobles as that was the only possible action to save himself while protecting Rai in shadows and making others belief he died.
Roctis as a part of the plan by the six traitors, joined Union so that they can use its resources when the final fight between traitors and Lukedonia starts.
Anyways, I dont believe all of them are part of Union.
^ i like ur theory about roctis especially trying to save rai's life I thought of that b4, that would make sense since rai is dying due to his power so probably he is trying to use union to learn how to strengthen rai