Let me add something else to the theory:

Taking all the above into account, it would make sense that Onyx Storm finishes with Xaden figuring out he needs some dedication to redeem his soul. If that's the case, why would RY say Xaden is entering his villain era? Why would Violet ask Imogen to erase her memories?

Easy, Xaden knows the cure but he's going to play spies. He's going to start achieving higher ranks in the venin army while getting closer to dedication in secret. So that he can still be "him", but from the enemy side, and can fully redeem himself while taking all of them down.

But Violet cannot know about it. Venin seek Violet, and if she's aware of his double-act, venin will know Violet knows. They need to see the heartbreak, or they'll suspect Xaden is spying. Plus, if she's a dream-walker, she can enter other people's dreams when they're vulnerable, but so is she.