Emma confessed “I was pleased with my own perseverance in asking questions, and amused to think how little information I obtained” (21). I was not suggesting Emma’s amusement was visible to others but that Emma’s confession of it showed that she knew Jane was only going to answer her questions with polite generalities. With such knowledge I feel the better part of civility was for Emma to accept Jane’s boundaries and pursue other topics. How is it polite instead of obnoxious to pursue an unwelcome subject with your guest?

Mr. Knightley’s idea that part of Jane’s reserve is founded in diffidence is rather puzzling if diffidence is defined as “lacking confidence… timid; shy” (dictionary.com) which it is but it is not the only definition. I am using definitions from Johnson’s 1824 Dictionary of the English Language, page 194 (Google Books) as a guide in determining what Mr. Knightley means by attributing part of Jane’s reserve to diffidence:

Diffidence – Distrust; want of confidence. Bacon. Doubt, want of confidence in ourselves. Locke.
Diffident – Distrustful; doubting others. Milton. Doubtful of an event; uncertain. Pope. Doubtful of himself; not confident. King Charles.
Diffidently – In a manner not presumptuous. Smart.

By her own (20) and Mr. Knightley’s account (21) Emma’s past attentions to Jane has been deficient. Perhaps Mr. Knightley feels Jane is diffident as in distrustful—it seems to me Jane would naturally be uncertain as to why Emma suddenly finds her worth attention. She cannot be insensible of Emma’s past neglect and in her place I would wonder if Emma’s sudden attentions are just a duty to be quickly put behind her or something more substantial. In this context Mr. Knightley’s suggestion Emma will be able to soon overcome that part of Jane's reserve arising from diffidence is really a compliment. He believes Emma’s sincere and steady attention will quickly remove Jane’s doubts:

"I always told you she was -- a little; but you will soon overcome all that part of her reserve which ought to be overcome, all that has its foundation in diffidence. what arises from discretion must be honoured." (21)