What Makes the Ending So Confusing
I think what makes the ending difficult to get at first is that the story we have been seeing is so symmetrical.
We meet three women. Each one is upwardly-mobile and insecure in her own way. There's the farm girl trying to figure out country club society; the working-class girl who married the boss; and the working wife who tries to elevate the family into the upper class through her own ambitions and career.
All three women are jealous of Addie. Addie seems to be the ideal that they are all trying to achieve. She is not just beautiful or wealthy -- she has "class," that ineffable quality which is what they are all striving for.
We are told that Addie always has perfect timing -- she knows exactly what to do and when to do it. So when they receive her letter just as they set off to "Kid Island" for the day, it creates emotional havoc that Addie would have predicted exactly.
At first, I thought perhaps Addie wasn't running off with any of the husbands. She knew about her friends' insecurities and how they all disliked her, so she had decided to move on. But first, she decided to play a prank on them that would ultimately be a great gift. Basically, she would have them all frantically worried about their marriages for a day. Then, they would gradually realize that their husbands were still there, and they would appreciate what they had so much more. Ultimately Addie would be strengthening their marriages while removing herself from the scene and diminishing her own social standing -- a great "classy" gift to all of them, in their parallel stories.
The fact that she *had* planned to run off with one of the husbands was so surprising to me that at first I didn't get it. The stories had all been so symmetrical up to that point. I thought Porter was making it up to soothe all the feelings at the table.
But after thinking through the tipped glass and the disappointed "Hi ho!" at the end, I'm pretty much convinced that Addie had planned to run off with Porter. He was about to become a "giant" and become even more incredibly wealthy, and he had clearly adored her. Despite his older age, less-debonair manners, and rough way of being, he could have been the best catch. But, ironically -- her own willingness to break up a marriage and run off with him might have lessened his regard for her. She was more alluring to him when out of reach and on a pedestal of "classiness." She had played it all wrong.