It does seem to me that Emma is in a position to do what she likes and to visit where she pleases. And certainly Harriet has become a welcome guest, even part of the family, at Hartfield, so I have never understood why that would change were she to marry Mr. Martin, who appears to be a respectable man. I think Emma is showing her own discomfort at being in unfamiliar settings when she says she couldn't visit the farm. As I think someone has already pointed out in this group read, Emma's life is very constrained, probably by her father's needs and his fears. She doesn't seem to visit her sister in London and has never even seen the ocean. Harriet, oddly enough, seems better able to adapt to new situations than Emma does