Emma and Elton
I think Emma's reaction to Mr Elton is also meant to show her snobbery at this stage, she would certainly be a very good match for Mr Elton, but there was not so great an inequality between them that he should not have aspired to her when her manners appeared to be encouraging. When John Knigtley warned her about her encouraging manners, it was after he realized that she did not know that Elton could be interested in her -- he didn't warn her not to be encouraging because Elton was not her equal, he warned her not to be encouraging if she wasn't interested in Elton.
Mr Knightley and Jane
Jane and both the Knigtley brothers were old friends -- re the conversation between John Knightley and Jane at the Hartfield dinner party. So probably Mr Knighltey considered that he was merely giving her the kind and proper attentions from an old friend. I admit though, that when I first read the novel some time ago, I did think of a Jane-Knightley match. Re-reading it after knowing the ending, I still think it a possibility that Mr Knightley might briefly have considered Jane as a possible match for himself -- either before he became aware of his feelings for Emma, or if he believed he had no hope of Emma because her affections were engaged elsewhere -- but that Jane's increased reserve certainly discouraged him. Jane was poor, but she was a gentleman's daughter and very well-educated and accomplished, so she would not be ineligible.
Mr Knightley and Harriet
I believe Mr Knightley never thought that his kindness to Harriet would give her any expectations of him. The inequality between him and Harriet was greater than that between Emma and Elton, or between him and Jane. That time at Donwell, when he seeemed almost to be asking her whether her affections were engaged, and then talked about farming, giving Emma a conscious look in case she suspected him of pleading Robert Martin's cause -- was perhaps because he did want to plead RM's cause!