Since we are given little enough of the dialogue from that evening, your take on whether Emma uncivilly presses Jane Fairfax on topics that make her uncomfortable is as valid as mine. But my take, says that Emma did not do so. For proof, I have Emma's behavior as a perfect hostess in every scene wherein we see her in that role, and Mr. Knightley's complete ignorance of either Emma's amused annoyance, or Jane Fairfax's determined reticence, despite watching both ladies (with great delight!) the whole night. I therefore believe the amusement Emma talks about is internal, and derives from the results of the conversation, not the anticipation of how she will react when Miss Fairfax withholds information.

Thank you for the definitions. Do you mind terribly that I still think reading it as "want of confidence in ourselves [..] uncertain [...] Doubtful of himself [...] not presumptuous" makes the most sense in context?

You see, I can not see Jane Fairfax as suspicious or distrustful of Emma's motives. She does not feel friendship for her, but is she assuming Emma will lead her astray, or use her for some ulterior motive, heretofore unhinted? If you mean, Jane Fairfax does not believe Emma's overtures will continue, you may be right (although, with discouragement from Jane herself, should they even be continued?), but would that cause the 'diffidence,' that Mr. Knightley insists is not the same as her reserve? And Emma says quite clearly that she does not see any diffidence. Surely, if they meant "lack of trust (in Emma continuing her attentions)," Emma would agree to it easily, since she also knows she should have been more attentive to Jane Fairfax during former visits.

I would think Mr. Knightley's assurance of Emma's quickly breaking down Miss Fairfax's reserve is a compliment, too, if I thought he has any idea what he is suggesting. If he sees no unfriendliness in Miss Fairfax's behavior, of course he thinks there is no impediment to his carefully laid plans for their future felicity. If Jane Fairfax is as little disposed to be intimate with Emma as your guess about her distrust implies, how on earth is Emma supposed to overcome it? Plus, it is heinously presumptuous of Mr. Knightley to assume that social barriers of polite reserve are only put in place in order to be broken down by repeated assaults. Miss Fairfax's reserve keeps Mr. Knightley from reading her, too, yet he thinks it does not. He will not even consider Emma's views to see if they are valid.

Think how happy he is in ch. 21, when he thinks they are going to be friends. How he approves Emma's actions the night before!

I am sure Miss Fairfax must have found the evening pleasant, Emma. You left nothing undone. I was glad you made her play so much, for having no instrument at her grandmother's, it must have been a real indulgence.

Then he quickly segues into his REAL intent:

[Y]ou will soon overcome all that part of her reserve which ought to be overcome...

Upon Miss Woodhouse stating that she thinks Mr. Knightley is misreading Jane Fairfax, he brings more pressure to bear:

"My dear Emma," said he, moving from his chair into one close by her, "you are not going to tell me, I hope, that you had not a pleasant evening."

"Oh! no; I was pleased with my own perseverance in asking questions, and amused to think how little information I obtained."

"I am disappointed," was his only answer.

He is not disappointed in her behavior to a guest: he has already complimented her on it. He is pleased that she is acting with more justice to Miss Fairfax's merits (as Emma is of herself). His disappointment is obviously in her refusal to fall in with his ideas of their becoming emotionally attached. When Emma relents with some compliments soon after, he looks gratified, and, I think, would have pressed the new opening with another attack, had not Mr. Woodhouse redirected the conversation.