Again: Right now, the Parshendi are actively trying to kill her people. The Heralds deliberately betrayed their oaths. I'm sure she'd be fine with a truce, if that were possible. But with the Fused forcing the Parshendi on, it's not.
The Heralds are basically traitors to her. Why should she have compassion for people who betrayed their oaths?
You call Jasnah cold and calculating, but you're okay with Amaram and T, both of whom are willing to sacrifice *innocent, harmless* people for their supposed selflessness? Sorry, I don't get why you like them but not Jasnah, who only wants to stop active enemies?
I feel like Shallan reverted to being very childish around Jasnah, and so Jasnah treated her like a child. Also, Jasnah isn't aware of a lot of Shallan's character development, so she tried to just pick up where they left off. Jasnah admits she was wrong to do that, and recognizes that action as a flaw in her personality.