I find many posters are often harsh on Edmund because of his infatuation for Mary. But I like Edmund much better than Edward Ferrars in S&S.

Although Mary is a very flawed character, she is also a complex character, not a simplistic mercenary character with no genuine feelings such as Lucy Steele who dropped Edward Ferrars as soon as she entrap his richer brother. In Mary's case, she decided very early on that she preferred Edmund to his elder brother, although Mrs Grant had at first planned to matchmake her with Tom --

chap 7
"to the credit of the lady it may be added that, without [Edmund] being a man of the world or an elder brother, without any of the arts of flattery or the gaieties of small talk, he began to be agreeable to her ... There was a charm, perhaps, in his sincerity, his steadiness, his integrity,"

and when Tom returned to MP in chap 12, she had --
"the fullest conviction, by the power of actual comparison, of her preferring his younger brother ... so far from now meaning to marry the elder ... that were he now to step forth the owner of Mansfield Park, the Sir Thomas complete, which he was to be in time, she did not believe she could accept him."

And at the end of MP, Mary was still living with a widowed Mrs Grant because she found it difficult to find "any one who could satisfy the better taste she had acquired at Mansfield, whose character and manners could authorise a hope of the domestic happiness she had there learned to estimate, or put Edmund Bertram sufficiently out of her head."

Fanny is an admirable character, but like all of JA's heroines, she is also human. I think Fanny is sometimes a little harsh on her views on Mary's possible future improvement if Edmund had married Mary. After all, six months at Mansfield was not enough to counteract the years of upbrinign by her embittered, unhappy aunt, the late Mrs Crawford, whose husband the Admiral was unfaithful to her -- Mrs Crawaford advised her niece and her nieces' friends to make mercenary marriages. Mary was cynical and disliked her uncle and spoke improperly of him uncle because of his infedility to her aunt.

Edmund may be deceived in some aspects of Mary's character, but he was not deceived in the fact that she had some genuine feelings for him, and it is a measuree of his character that he managed to make such an impression on such a woman, that she found it difficult to put him out of her head even after his rejection of her (and Tom's recovery). Edmund was not Edward Ferrars who was completely deceived in Lucy.