I feel that Emma is being very manipulative here, if she had condemned the letter outright as badly written, even Harriet might feel that she's not telling the truth and be less likely to trust her so completely.

I pity Harriet a great deal! Not only because Emma is unlikely to raise her from her class, but because I see her as a simple minded girl who is capable of some warm feelings, and Emma is messing up her feelings. As for loyalty, I think poor Harriet is torn betwen the Martins and Miss Woodhouse -- she sees Miss W, "so great a personage at Highbury", as having been so kind to her, she lacks self-confidence to set up her own judgment and opinions agaisnt those of Miss Woodhouse.

Emma is a manipulative liar here, she claims not to give Harriet her own opinion about Robert's proposal when in effect she is indicating her opinion -- her disapproval -- only too clearly! To strengthen Harriet's refusal, she then more or less blackmails Harriet by saying she could not have visited Mrs Robert Martin -- in effect forcing Harriet to choose between RM and herself.

I think Emma show herself at her worst in this part -- if we do not know she is the heroine, her conduct here is more like that of a villainess!