Harriet enjoys Hartfield and the elegancies therein but that is exactly as Emma has been training her up to do. I disagree Harriet views Emma as a stepping stone to position and fortune. To me a stepping stone implies something is temporary on the way to something better and I don’t see any evidence Harriet wishes to loose Emma’s friendship or leave her behind and go on to greater things:

"You could not have visited me!" she cried, looking aghast. "No, to be sure you could not; but I never thought of that before. That would have been too dreadful! What an escape! Dear Miss Woodhouse, I would not give up the pleasure and honour of being intimate with you for any thing in the world. …Dear me! How should I ever have borne it! It would have killed me never to come to Hartfield any more!" (7)

I do not think Harriet is being guided by materialism to reject Robert Martin rather it is persuasion in the form of Emma’s relentless devaluing comparisons combined with Harriet’s reverence for Emma. Harriet was introduced to Miss Woodhouse by Mrs. Goddard (3) so along with her cleverness and winning ways Emma also has the recommendation of Harriet’s guardian to spur her admiration and desire to please. I am sure Harriet has been told Emma is the cats meow and she ought to be and is grateful for her notice. My point is Harriet has every reason to admire and completely trust Emma just like Bingley to Darcy:

On the strength of Darcy's regard Bingley had the firmest reliance, and of his judgment the highest opinion. (4) …His diffidence had prevented his depending on his own judgment in so anxious a case, but his reliance on mine made everything easy. (58)

Harriet appears to mirror Bingley in the “easiness, openness, and ductility” (P&P, 4) of her temper but unfortunately falls short in being far more deficient in understanding than Darcy’s dear friend. Of course technically Harriet has free will and could have accepted Mr. Martin and weathered Emma’s disapproval but IMO she has not the will or confidence in her own opinions to take such a stand against or the desire to disappoint her new best friend. I think Harriet has pretty much always been a girl who allows others to guide her opinions and decisions after all she readily accepted the fact she could never know the identity of her parents:

Her first attempts at usefulness were in an endeavour to find out who were the parents; but Harriet could not tell. She was ready to tell every thing in her power, but on this subject questions were vain. Emma was obliged to fancy what she liked -- but she could never believe that in the same situation she should not have discovered the truth. Harriet had no penetration. She had been satisfied to hear and believe just what Mrs. Goddard chose to tell her; and looked no farther. (3)