I think this absolutely works on his guilt. He's finally getting a taste of that sh!t sandwich he made by walking out on you, and it's vile.
I doubt he'll be gone a year. It's easy to walk away when you're convinced you're right, but he knows he's wrong and it's eating at him already. When OW can't fill the hole in his heart left by Baby Grand (and she won't), he'll be back.
Go dark on him, work on you, but send pictures to MIL. If she's Italian, Catholic, or Jewish, she'll work the guilt angle for you, demanding to know how he can stay away when his daughter needs him so much to be there.
