I have to say I have not read MSND or even seen an adaptation of it so I am responding only on the situation as you describe it but I think slipping the comparison in Emma is terrifically fine writing on JA’s part. (;D) I think you are right Emma’s comparison shows she does not understand the character’s situation she is quoting which probably means she did not read all of the play. She is comparing on half-baked information which sort of reflects how I see her matchmaking efforts in general—she does not understand men and women or relationships, has no experience herself but is still trying to handpick husbands and wives. I think she feels her efforts are putting Harriet and Mr. Elton on the course of true love and she has had to weather the rough patches Mr. Martin and Mr. Knightley have put in her path, their pre-arrangements, and applies the quote to her and her efforts. I think it would be most appropriate for Mr. Martin to say “the course of true love never did run smooth.” Poor guy! He has done so much and gained nothing but a sad heart—I love how JA made his courtship so much like Lysander’s and I think Harriet was sort of bewitched by Mr. Martin, all the Martins really, their home and even their cows. I also do not think Emma sees herself as a person who would split up two people attached to each other but that seems to be just what she is doing. Thanks for pointing this out Barbara because otherwise and sadly too, it would have whizzed right over my head. (;D)