I think Mary's attention to Fanny in this scene is her best moment and shows there is some goodness inside there.

Mary isn't my favorite. She's too manipulative for me. I got to wondering if "almost purely governed" meant the opposite - barely governed, but ultimately I think that her motives were mostly good, but she is aware it helps her standing with Edmund.

I think her initial reaction was very sincere...
but Miss Crawford, looking for a moment with astonished eyes at Mrs. Norris,...immediately said, with some keenness, “I do not like my situation: this place is too hot for me,”

I looked up keenness in Johnson's dictionary so I could be clear on what that meant- sharpness, asperity, vehemence. I think it shows she was shocked enough to respond in a harsh manner.

It's also interesting that the family is so inured to harsh treatment of Fanny that they all just sit there like lumps. Edmund may have been too angry to speak, but I think it was high time someone did. And why should Fanny be seen as weak for not speaking up for herself, if the rest of the family acts like this is normal? It's odd that the baddies- Mary and her significant looks to her brother- are the ones who diffuse the situation. Ugh. Sorry- it gets me a little fired up.