I didn't know it was the mask that was controlling Siggy; I thought the mask thing was just in keeping with the style of Karnak's followers, as in things that cover their faces/eyes (the blindfold for TIM, mask for Siggy/Evil!Jayden). When was it established that the masks were actually controlling entities?

Besides, we're working on Mookie's Razor here: Whatever makes sense or would be awesome, is not what's actually going on. Whatever makes the least sense or would only be awesome to Mookie, is what happens. I'm willing to bet it isn't anything so elaborate as "the mask is controlling him, and now he's free", and Siggy's still screwed because "Siggy Will Not Be Redeemed In This Arc Or Ever You Guys, Stop Liking Him!".