Does anyone else feel that Stormfather's behavior, goals, and motives with Gavilar seem odd compared to the Stormfather that we've seen interact with Dalinar? I really feel like this is an imposter... Stormfaker. The list of suspects is pretty short; they'd have to have a lot of information, the ability to fool Gavilar and steer clear of Heralds. I'm leaning towards one of the Heralds (probably Paliah) for this. It's within the realm of possibility she became a Truthwatcher like Nale became a Skybreaker. We don't have examples of what a full-fledged Truthwatcher can do with Illumination, and while I agree that "trickery" with light and sound would fall under the province of Lightweavers, visions could be a Truthwatcher's domain. As for motive, perhaps the plan was to get substitute Heralds to free up the originals from the Oathpact. Perhaps it was something more in line with Truthwatcher beliefs.
Also, the way Stormfather is depicted as a shimmering form. That's new?
And lastly this:
We know that Heralds can feel when one of their number dies. In his conversations with Dalinar, Stormfather has said he doesn't really keep tabs on Heralds (other than knowing about Ishar). That Stormfaker immediately knew when a Herald (probably Chanarach) died, makes this theory more likely. I think whoever this Stormfaker was, their initial plans went up in smoke with this.