I've been entertaining the idea that Chanarach is not Shallan's mother. Chanarach is Shallan's father. Feel free to poke holes in that theory. Here's the breakdown:
We know the Heralds’ madness is tied to their heraldic attributes being twisted in some way. Chana’s attributes are “Brave” and “Obedient.” Lin Davar is a paranoid man, controlling to the point he’s obsessed with being obeyed and enters a blind rage whenever someone defies him.
Heralds (and other highly Invested entities) have been shown repeatedly to be more resitant to physical damage. When Shallan kills her father, she uses the blackbane her brother Wikim gave her years ago. Blackbane is stated several times and by different characters to become more potent over time. Yet, when Shallan poisons her father, not only does it not kill him and only paralyzes him, he actually begins to recover from it, which is what prompts her to strangle him instead.
And when she strangles him, she uses her necklace, which for Brandon felt the need to specify on-screen was made of aluminum. I feel like this is probably relevant.
Do we know for certain Chanarach is a woman? We have the portraits, which we know aren’t accurate, and that’s about it. People who know Chana (Hoid, other Heralds, Shards and spren), never refer to her as a “she”. The Stormfather doesn’t contradict Gavilar when he does, but that doesn’t mean anything, given that we know the Stormfather can lie by omission. And even if she actually is a woman, we don’t know the process used to give the Heralds a body when they return. Something could have gone wrong.
We don’t know which Herald dies the night of Gavilar’s assassination. We do know that there were several of them in the palace that night, and that Szeth was on a killing spree at the time. In fact, the unknown Herald’s death happened only moments before Sadeas came to warn Gavilar.