because she wants to be what Emma is...the first lady of Highbury.

>* They each want to be first and foremost.

The difference is that in Emma's case *she is* (rather than wants) while it is Mrs Elton who 'wants'.

>* They each indulge in a great deal of self-deception

That's quite true though there are others too like Mr Knightley.
Seems to be a very common thing in Highbury.:-D

>* They each love to run interference in others' lives and each has taken it upon herself to supervise the life of protegee.

Of course, if Emma has a Harriet then she too *must* have someone so that she can do the grand thing of guiding like an important lady.
She's copying.

>* I believe (I think I have this right) they each would like to own the ability to play well but is doing nothing about it.

Emma plays quite well BUT not as well as Jane and that she accepts. Mrs Elton thinks she plays well, but like all other married women "has given it up."

Emma is the one *she* has to beat in importance, not Mrs Weston, not Jane and definitely not Harriet!!!

She even wants to compare Hartfield to Maple Grove, so why not herself to Emma!!!