I agree about Emma's praise in chap 18 as a trigger to jealousy -- and a beginning of Mr Knightley's realization that he himself was in love with her. This fits in with my opinion that Mr Knightley was unaware of the nature of his affection for Emma in chap 5 -- when he said to Mrs Weston that he would like to see Emma in love with a proper object and in some doubt of a return (although more romantic readers have a different opinion).

Mr Knightley also had a rational reason for not thinking well of Frank Churchill's delays adn excuses -- and in chap 51, when reading and discussing Frank's letter of explanation about his engagement --

'"He trifles here," said [Mr Knightley], "as to the temptation. He knows he is wrong, and has nothing rational to urge. Bad. He ought not to have formed the engagement. 'His father's disposition:' -- he is unjust, however, to his father. Mr. Weston's sanguine temper was a blessing on all his upright and honourable exertions; but Mr. Weston earned every present comfort before he endeavoured to gain it. Very true; he did not come till Miss Fairfax was here."

"And I have not forgotten," said Emma, "how sure you were that he might have come sooner if he would. You pass it over very handsomely; but you were perfectly right."

"I was not quite impartial in my judgment, Emma; but yet, I think, had you not been in the case, I should still have distrusted him."'