The affection of the whole family, the warm attachment of Miss Campbell in particular, was the more honourable to each party from the circumstance of Jane's decided superiority both in beauty and acquirements. That nature had given it in feature could not be unseen by the young woman, nor could her higher powers of mind be unfelt by the parents. They continued together with unabated regard however, till the marriage of Miss Campbell, who by that chance, that luck which so often defies anticipation in matrimonial affairs, giving attraction to what is moderate rather than to what is superior, engaged the affections of Mr. Dixon, a young man, rich and agreeable, almost as soon as they were acquainted; and was eligibly and happily settled, while Jane Fairfax had yet her bread to earn. (Chapter 20)

I don’t think Mr. Dixon was attracted to Jane or her to him. I read the narrator’s description of Mr. Dixon’s affection being bestowed by chance and luck on Miss Campbell to mean that it was not contrived in any way but a natural attachment. Jane is superior in beauty and acquirements to her friend who is described as moderate in the same. I read chance ‘giving attraction to what is moderate (Miss Campbell) rather than to what is superior (Jane)’ in Chapter 20 as referring to beauty and acquirements. This is an indication to me that Mr. Dixon preferred Miss Campbell’s moderate personal merits over Jane’s superior personal merits for whatever reason. At least some of Miss Campbell’s attraction may be that she is a generous and warm person. The narrator says Miss Campbell was not unaware that Jane outshone her in physical beauty yet they ‘continued together with unabated regard’ until Miss Campbell’s marriage and it seems also afterwards since both the Campbells and Dixons encouraged Jane to attend them to Ireland and later write to invite her again.

IMO if Mr. Dixon attached himself to Miss Campbell because she was a better catch then his choosing her would not be chance and luck but rather sense and prudence or on the other hand calculation and greed. Also if the narrator was referring to dowry and connections rather than beauty and acquirements then Miss Campbell would be deemed superior and Jane moderate or less. I also think the Dixons’ being ‘eligibly and happily’ settled means there is no obstacle to their happiness such as a prior attachment to Jane on his part. From Dr. Johnson’s:

Eligible: Fit to be chosen; preferable.

Ref: A Dictionary of the English Language by Samuel Johnson, John Walker, Robert S. Jameson, 1828.

I think Frank makes the most of Emma’s speculations about Jane and Mr. Dixon because he wants to prevent any suspicion of his attachment to Jane and because he is not quite as considerate of Jane or his friend, Mr. Dixon, as he ought to be. I agree with you. I think Frank’s failure to visit the new Mrs. Weston and his father is almost pure selfishness because he preferred to visit Jane instead only, finally making his way to Highbury after Jane arrived. I say almost pure selfishness because I think he knew Jane also wanted to see him. I think Jane was glad to visit her aunt and grandmother but the real reason she preferred to visit Highbury rather than Ireland was to meet with Frank. I think Jane and Frank really did form an attachment to each other at Weymouth so I don’t think Jane was ever attached to Mr. Dixon.