- from tWoK Chapter 72.
Jasnah here is talking about spren being linked to Radiant abilities. The above quote would indicate that Jasnah had assumed Shallan of being in the same order as her, but revises that opinion when she see's Shallan's spren.
I'm not entirely convinced that Jasnah assumed that Shallan was in the same order as her. Her phrasing is "I had assumed...". This means that she had made the assumption at a previous time, and was now looking at the assumption again and discarding it. Shallan only revealed that she could Soulcast minutes before. I think if Jasnah had been referring to to Shallan's order, she would have said "I assume that you... but no," indicating that she's making the assumption and correcting it mid-thought. The "I see now" only reinforces the idea that Jasnah's assumption was made previously. I'll admit that she could be referring to an assumption made minutes before, when Shallan revealed that she could soulcast. But it feels wrong to me. I think she's referring to an assumption made much earlier than that. What it might be, I have no idea.
Back to the topic - what do you see as the defining moment of Jasnah's character in this book? I don't know about you, but I see it as the scene where she kills those men. And in that scene, the two qualities that define her the best are Just and Confident. Even without knowing about the Ars Arcanum, I think I would pick those two qualities for that scene. Yes, she's Learned and Careful, maybe she's Wise and Giving, but I just feel that Just/Confidence are at the core of her character more than anything else (also note that this is one of the only scenes we see her soulcast). I find it really difficult to argue with my gut on this one. And the fact that every other piece of circumstantial evidence we've found supports Jasnah being in Order 2, and I think this outweighs the restriction she has to be in order 5 or 7.