Emma’s neglect of Jane seems to be a thread in her conversation with Mr. Knightley in Ch. 21 and for this reason the idea Jane’s diffidence is founded on distrust appeals to me. That he may connect Jane’s diffidence to Emma’s neglect seems reasonable and natural in this conversation. I think the distrust Mr. Knightley may attribute to Jane is as you said doubt Emma’s overtures will continue, that they are not just show. In such a situation it seems logical a sensible girl like Jane would be reserved and cautious in investing emotion and confidences in Emma.

Emma may be right that Jane is not diffident. Of Johnson’s definitions, lack of self confidence, distrust, doubt of others or an event and not wishing to be presumptuous there is not much to tell. I don’t see that Jane lacks self-confidence. My best evidence for distrust I do not claim to be infallible. Lastly I don’t know why Jane would feel it presumptuous to answer Emma’s questions with her opinions—that is what Emma requests.

I think Emma did press Jane but it did not have the appearance of incivility. In saying she understands Mr. Knightley’s comment “No …you are not often deficient; not often deficient either in manner or comprehension” (21) well enough is not Emma admitting where Jane Fairfax is concerned she has not always been a perfect hostess? Mr. Knightley saw “only proper attention and pleasing behaviour” (21) between them at Hartfield (20) but he cannot read their minds. He takes the appearance of a pleasant exchange at face value—I daresay it is no stretch of their powers that Emma can ask question after question and Jane can answer one after the other without displaying any degree of unpleasantness. I think Mr. Knightley’s ignorance of Emma’s amused annoyance is a testament to her ability to hide it and his desire to see the best in her but is not evidence she did not press Jane for her opinions.

IMO when Emma says “I was pleased with my own perseverance in asking questions, and amused to think how little information I obtained” (21) she is speaking of the past. She says “I was pleased…amused…obtained” which I interpret as Emma describing what she felt while questioning Jane the previous evening. It seems to me an admission of pressing Jane for information. I think Mr. Knightley is disappointed to learn Emma’s only pleasure in Jane’s company was in her own little game against the lady’s reserve. It shows Emma gave proper attention but did not offer the hand of friendship.

I agree with Gianni in post 49876 that Mr. Knightley wants Emma to do her duty by Jane. I also think Mr. Knightley wishes them to be friends. I don’t see that Mr. Knightley is trying to force Emma into friendship with Jane but he is encouraging it and if Emma would listen perhaps some real damage might be done to the “intellectual solitude” (1) she has faced since Miss Taylor left Hartfield. Her friendship with Harriet has produced little good and much to deplore. Jane is more Emma’s intellectual equal and perhaps with her Emma would find less destructive projects than matchmaking. There is a glimmer of hope in that Emma is at least of a mind to do her duty by Jane and perhaps that will lead to more:

"Of the same age, and always knowing her, I ought to have been more her friend. She will never like me now. I have neglected her too long. But I will shew her greater attention than I have done." (34)