Edward's motive

That is what I wondered too.
The storyline in his novel was so dramatic, life and death. THAT story and characters were gripping. That should have been it's own movie, without the art dealer and novelist story.
The storyline in the 'real world' was so banal. Girl divorces boy, so 20 years later, boy gets revenge by standing her up for dinner? Is that all there is? I kept expecting something bigger to happen.

It was beautiful to look at, all Tom Ford movies are beautiful, but the 'real world' characters and plot were paper thin and I did not care about them at all. Poor little rich girl can't sleep and got stood up for dinner. Is that really all there is??

No.

The stand-up was to send the final message to her that the novel she just read was symbolic, representing the wife and daughter that Edward lost, that she took from him.

Recall that throughout the film, she couldn't sleep, wracked with guilt about the way she dumped him (and the abortion). She said as much to Alessia (the woman she confided with at work). And it had been 20 years. Add all the guilt about her becoming her mother, which was is finally what caused her to dump the man she loved. Gold-bricking Hilton was there to help her along with that and now she knows that he's not faithful to her.

Then came that last dinner dinner. Watch her face through those scenes (Amy Adam's did some incredible acting in a lot of those scenes with Edward). She was smug at first, happy with the fact that he agreed to meet with her. She believe it's a sign that he is now beyond that, which means to her that she can relax her guilt, get some sleep, be a happier person. And maybe she's not like her mother after all.

Then she's unsure, nervous, orders another manhattan.

Then she knows.

That dinner was as symbolic as the novel was. It was the epilogue.

Speck.