I dont think that Emma's dislike of Jane is mainly to do with jealousy. I think it is just htat their temperaments are incompatible, but I DO think that she gets irritated that Jane is so frequently praised. She knwos that she should not, and she tries ot overcome it by paying Jane compliments, but when she has to listen to people praising her like Miss Bates, and even has a twinge of fear that Mabye Mr Knightly is rather fond of Jane, her irritation and jealousy get hte better of her, and she "fancies" unkind things about the girl or gets fretful and irrated. I certianly DONT see JA's use of the words unfeigned etc in relation to Emma's regret as ironic. I think that seh DOES regret that she is not steady enough in her practising to be as good as Jane, but all the same her self esteem does not depend on accomplishemnts but on her belief in her own intelligence and her high social position. So she's not likely EVER to be devoting herslef passiionately to her music. However it does not mean that she does not at times regret her own lack of steadiness in practising her music, reading seriously etc.