>more than one gentleman has rendered Harriet a service in the recent past and it might have been better if she had named names.

Emma’s imaginations revolve around heroes rescuing heroines from ‘danger’. Service rendered which does not involve rescuing from danger is not ‘imaginist material’.
IIRC there doesn’t seem to be any other such.

>her grubby little Dixon/Jane fantasy,

With Jane her imagination goes further…of that of intense suffering with pangs of disappointed affection, and shedding of tears (CH.27)
A typical scenario of a heroine from the period of romanticism – all about unrequited love and suffering etc etc.
In fact she sees Jane as getting away from that situation by coming to Highbury instead. She does not see Jane involved in any grubbiness.
IMO readers of JA times would have seen this immediately in Emma’s imaginings rather than any ‘grubbiness’.

One incidence at Box Hill changes Emma.
IMO it doesn’t need several chapters to find out if Emma will learn anything to change these applications of fantasies as facts.
All she needs is to have her fantasies proved wrong when thought of as facts.
But I doubt she will change into a non imaginist. She’ll just stop taking them as facts, I should think.

>Then comes her insult to Miss Bates, and - what is actually worse - she promptly forgets about it

Of course she forgets about it!!
If she had deliberately meant to insult she would remember it as an act planned to insult Miss Bates.
But she meant to be witty – and wasn’t.
It was an act of thoughtlessness, and a thoughtless thing cannot be thought about and remembered unless reminded of it and told how it really was.
Thanks to Mr. Knightley for that.