Well I do find it hard to believe that Edmund is so unaware of how unkind Mrs N is or can be, and how little reason Fanny has to like or trust her kindness. True he is a male and of hte Bertram family so Mrs Norris treats him properly but he can't IMO not noitce that she does not treat Fanny well... even a little Later Julia will make a remark to Mrs N about how unfair she is to Fanny....
So I think if he doesn't notice how unkind she is, It is a fault in him as it is in Sir thomas too. and if he's really concerned about Fanny he coudl speak to his father and say that he doens't think that she'll be happy or well treated with Mrs N and wouldn't it be just as well if she stayed at Mansfield...