I read the chapters you quoted rather differently than you do. Emma is not the kind to obtrusively question a guest of Hartfield. She asked leading questions of Harriet and Miss Bates, yes, but they are comfortable around her, and obviously did not mind. Her questions by no means constitute an impolite barrage in either case. With Harriet, she was drawn out over the course of many meetings. With Miss Bates, one question opens the flood-gates of information.

Do you not think Mr. Knightley's approval of the evening would have been less, had he seen Emma overstepping decorum in her attentions to Miss Fairfax? I do. I also think that the lady of the manor, a part Emma has played for many years now, would be quite capable of hiding her amusement at her own frustration for the space of an evening. No, I think Emma expected a more forthcoming conversation, and hid her disappointment that it did not appear.

Mr. Knightley assigning diffidence to Jane Fairfax is also a mystery to me, since she seems a very composed, confident, young lady. Your definitions of "distrust; want of confidence" ring another bell with me: Jane Fairfax does not SEEM to want self-confidence (which is what I took your definition to point to, until you defined 'confidence' to mean 'intimacy'), and Mr. Knightley is being overly generous to impute her silences in Emma's presence to being uncertain of herself. Perhaps she does 'distrust' Emma; however, that is hardly likely to endear the two to one another!