The thing is, your D's birthday party has nothing to do with a divorce or marriage or the price of coffee. It just IS. Why not celebrate it as it should be celebrated. If his family feels awkward then that says a lot more about THEM than it does about anything else.
I also believe that your H enjoys stringing you along. One minute he's wearing his ring knowing you're noticing and the next minute he's screaming into the phone "because we're getting divorced!". He knows you're strapped into the roller coaster car.
