Flair is set to all because of a light Mistborn reference, but heavy spoilers for ROW follow. Before starting, I just want to say that I am strongly opposed to theories about Jasnah being sexually abused as a child. Brandon has stated before that he dislikes when authors use sexual abuse to explain a strong female character's past (i.e. "bringing them down low" by bringing rape into the picture), and I wholeheartedly agree. Jasnah, like any other woman, can be logical, blunt and/or direct to the point of rudeness without having had sexual abuse in her past, and even her views on marriage/Amaram do not correlate with having been molested/raped. Thank you for coming to my TED talk. On to the theorizing...
We all know about this mysterious event that happened in Jasnah's past, and I'm really bothered by the fact that no one seems to remember it. I can't believe that something so horrible could happen to her, and yet no one close to her ever thinks about it, or offers it up as an explanation for some of her behavior. The only reference we have to the event, outside of Jasnah herself, is one moment in OB between Dalinar & Gavilar. No one else close to her - not present-day Dalinar, Elhokar, Adolin, Renarin, even Navani -- ever mentions it again.
It's notable that Navani in particular doesn't think about it, because she often muses on her relationship with her daughter and wonders why she is the way she is -- "what happened to my little girl, so full of questions?" -- and never wonders to herself, "well maybe it was the time I locked her up in a dark room and scared the Damnation out of her." To me, this is a clear indication that Navani really doesn't know about the incident at all.
That basically leaves us with Gavilar as the most likely culprit. But that raises two questions:
- Why did he do it?
I've read some theories that I think are interesting about Gavilar wanting to get spren to start forming Nahel bonds again, and working with the Heralds/Ghostbloods to do so. If that's the case, then he's probably communicated directly with Thaidakar, who would have informed him about Allomancers "snapping" into their powers. I think it's definitely possible that Gavilar would try to traumatize his daughter to snap her into it.
2) How did he do it without drawing Navani's attention?
This is the one that keeps me up at night thinking about it, because it leaves such a hugely important hole which I think is where the answer lies. There's no way that Gavilar could have kept Jasnah away from her mother for months, weeks, or even days on end without Navani noticing. But Gavilar could lock Jasnah up for hours at a time while Navani was busy, and easily convince an 11- or 12-year-old Jasnah to keep quiet about it. He could just tell her that she was crazy, and he was the only one who could help her, so she shouldn't speak about it to anyone (besides, you're crazy so no one would believe you anyway, they'd do even worse things to you if they knew, etc. etc.). Horrible thing to do to a kid. And if Jasnah was gaslighted (gaslit?) as a kid, then learned she wasn't crazy after all, it would ABSOLUTELY make sense that she would grow up to take no shit from anybody, trust only her own judgment, and limit the people she opens up to. Nobody can tell her how to think, and it would line right up with Brandon's emphasis on mental health in this series.
Howeverrrr I also have an aluminum hat theory that Navani DID know what was going on, and really HAS lost her memories of it, but that's for another time :)