…your insight into the subject of Harriet that interests me too.

While I agree with you regarding Emma, whose comments about the farmer vs gentleman comparisons seem to get stronger in direct proportion to the threat to her plans, I can’t agree about Harriet.
> she is sweet, docile and not clever or unduly intelligent,

This is how* Emma* sees her. or is willing to see.
Harriet has her own POV of things which she can defend very well.

What’s stopping her from following her heart *and* mind in this particular ‘case of marriage’ is my point all along.
She doesn’t think any less of Robert.
She had thoughts of marriage to him, and encouraged (so Robert Martin proposed)
She doesn’t agree to all Emma says
Has conscience which has been pricked at times
Shows her loyalty by not changing her view about him

So this was the right time for her to say as she does to Emma in another conversation;

"Ah! so you say; but I cannot believe it."…about RM. Or
"Dear me! it is so odd to hear a woman talk so!". :-D

IMO she goes along because she too has ideas of belonging to the gentry.

I don’t agree with the older younger comparison. She’s about 4 years younger, hasn’t had a life as sheltered as Emma has had, interacts with far more people, has gone to friends on holiday, gossips with the school teachers (most probably older), and has perhaps learned to fend for herself.