I thought that the ending gives us another example of superb irony. Mrs Clay and William Elliot have been at war for the entire novel. Mrs Clay knows that Sir Walter is clearly dumb enough to marry her for all she has to do is to flatter him outrageously and that Elizabeth is dumb enough to ignore that possibility until it is too late. Mr Elliot knows that by marrying Anne, he will always be around to circumvent Mrs Clay's designs. But Anne doesn't co-operate.
Thus, William Elliot buys off Mrs Clay by making her his mistress:
The news of his cousin Anne's engagement burst on Mr Elliot most unexpectedly. It deranged his best plan of domestic happiness, his best hope of keeping Sir Walter single by the watchfulness which a son-in-law's rights would have given. But, though discomfited and disappointed, he could still do something for his own interest and his own enjoyment. He soon quitted Bath; and on Mrs Clay's quitting it soon afterwards, and being next heard of as established under his protection in London, it was evident how double a game he had been playing, and how determined he was to save himself from being cut out by one artful woman, at least.
But Mrs Clay may triumph in the end:
Mrs Clay's affections had overpowered her interest, and she had sacrificed, for the young man's sake, the possibility of scheming longer for Sir Walter. She has abilities, however, as well as affections; and it is now a doubtful point whether his cunning, or hers, may finally carry the day; whether, after preventing her from being the wife of Sir Walter, he may not be wheedled and caressed at last into making her the wife of Sir William.
That paragraph is vintage Austen irony!