I cannot call contocting a fantasy involving supposed adulterous yearnings (I acquit Emma of imagining actual adultery, she does seem to draw the line there) between a newly married man and a young woman who is his bride's beloved friend, almost a sister, on no real evidence at all, anything positive. Not to mention getting so carried away by it as to thoughtlessly share it with a comparative stranger. So I will stand by grubby, unless you would prefer nasty or spiteful. And I don't think Emma sees Jane as a Romantic (in the literary sense) heroine - rather, at times, Emma seems almost to gloat over Jane's supposed moral shortcomings, as when in Chapter 28 she thinks "This amiable, upright, perfect Jane Fairfax was apparently cherishing very reprehensible feelings". This on the basis that Jane smiled at one of Frank Churchill's remarks. We don't know why Jane smiled, and there could have been any number of reasons, but Emma automatically assigns the one that best fits her less than amiable fantasy. Though she is sorry she ever mentioned it to Frank, and she does have moments of genuine pity for Jane, she is still rather unhealthily wedded to her belief in the truth of her fantasy. As you say, if it is proved that her fantasy is wrong she will abandon it - she will have little option. I have no problem with Emma continuing to imagine things if she can learn to distinguish her fantasies from fact and stop trying to make their objects act them out or repeating them to third parties as if they were facts. Most people learn to distinguish fantasy from fact long before reaching the age of nearly 21.