I like this analysis of Mary's character, and think you put it very well Bridget D. Mary is insensitive to people who's ideas don't chime with her own, but that doesn't make her cruel, IMO.

To me, cruelty means deliberately causing pain to another person. Henry is cruel when he seduces women whilst he has no intention of reciprocating their feelings.

When Mary colludes with Henry over the gift of the necklace she does not intend to cause Fanny pain - she thinks that a little love may animate Fanny and do her good. And when Mary expresses views which don't chime with Edmund's or Fanny's ideas, she has no intention of inflicting pain when she does so - she is simply telling it as she sees it. Mary does not feel pain when Edmund expresses opinions which differ from hers, so she does not expect him to feel pained by her opinions.