---(For example she knows Elinor would be too honorable to betray a confidence and uses that against her.)

Excellent point. People quite without honour don't hesitate to take advantage of it where it serves them. Willoughby, for instance, is so dishonourable it would make a Klingon sick to be in the same room with him for ten minutes, and it's part of the reason he loathes Brandon: he sees in the Colonel what he lacks, and not just material possessions, either. Brandon, like Edward, is a man of honour who comes from circumstances that would not seem to promote that quality---perhaps Brandon's mother, like Edward's father, could be the source?