Your final point, I mean - the first is spot-on. Harriet is not "little concerned" about having hurt Robert Martin's feelings, indeed she was described in Chapter 7 as being very much concerned at making him unhappy even as Emma dictates, er, advises her on how to word the letter refusing his offer. Mr Elton does not give a toss about having possibly hurt Harriet. To be fair to him, he does not need to feel guilty because he has not misled her in any way - from his viewpoint she just happened to be there while he was courting Emma. The blame for Harriet's delusions rests squarely with Emma, as she feels herself. On the other hand, a better man would feel concern that he had, however inadvertently, caused pain to a blameless girl.