Why do you say that Emma is not investing equal time in persuading Harriet to no longer be in love with Mr Elton? JA tells us that She [Emma] got her to Hartfield, and shewed her the most unvarying kindness, striving to occupy and amuse her, and by books and conversation, to drive Mr. Elton from her thoughts. (chpt 17).

This sounds to me like investing time in persuading Harriet she is not in love with Mr Elton.

Emma does enlighten Harriet about Mr Elton: Harriet was further unfortunate in the tone of her companions at Mrs. Goddard's; Mr. Elton being the adoration of all the teachers and great girls in the school; and it must be at Hartfield only that she could have any chance of hearing him spoken of with cooling moderation or repellant truth. (chpt 17 again)

I suspect what you may be criticising is Emma not spending hours and hours talking about Mr Elton. I do not think this would be an effective way of persuading Harriet out of love with Mr Elton. Instead, changing the topic and distraction are very good ways of changing feelings in my experience. One should spend a certain amount of time working through the feelings, but I find there's a point where I am just going around and around in circles, and in that case trying something else, be that going for a run, or taking up a new hobby, or making an arrangement with a friend to go out, makes me feel far better. I learnt this from my father. And of course, in the long-run, if we are to get over anything, then we must eventually start thinking about other things, so starting to practice this makes sense as a change process.