Whenever a group read is coming up I try to do some background reading. One of the things I read---and I believe I’ve seen this before---is that the ending doesn’t work/is weak because Austen provides this sudden marriage of Eleanor’s to this never mentioned Viscount to make it possible for Catherine and Henry to get married. I think JA knew exactly what she was doing. IMHO, I think it’s evident that she’s having a bit of fun mocking contrived endings by purposefully contriving such an ending and thereby adding herself to the list as well:

“The marriage of Eleanor Tilney,... is an event which I expect to give general satisfaction among all her acquaintance....Her partiality for this gentleman was not of recent origin; and he had been long withheld only by inferiority of situation from addressing her. His unexpected accession to title and fortune had removed all his difficulties;... Concerning the one in question, therefore, I have only to add — aware that the rules of composition forbid the introduction of a character not connected with my fable...”

After purposefully breaking the rule, almost with a smirk she adds an oh-yeah-I forgot-to-tell-you earlier, “...that this was the very gentleman whose negligent servant left behind him that collection of washing–bills, resulting from a long visit at Northanger, by which my heroine was involved in one of her most alarming adventures.” (31)