I found in my sitch that when my H had absolutely no sane reason to be doing what he was doing (walking away from his wife and life into the arms of a nutso ow), he decided to be angry with me. I guess it helped him detach easier, I don't know. I do know that for me, detachment was all that saved me during those times. It doesn't come easy, I know. Just remind yourself on a continuous basis that your H isn't the man right now who you fell in love with, married, brought children into the world with and built your life with. Therefore, you just cannot take to heart the mean things he does and says. Ignore, ignore, ignore.
Big kudos to you for not texting him when he acted ugly dropping your son off. Remember in the past I've told you that you often have to treat the MLC'er/WAS as a toddler or a tween who is throwing a fit...just walk away and eventually they tire of throwing the fit for just an audience of one (them).
I know it will seem terrible if your H chooses not to go to the party, but I promise you it will not be the end of the world. There are so many events I had to attend without my H, and I always made sure I looked smokin' hot. It wasn't for him (heck, he wouldn't even be there!), but it was for ME...and trust me, word always got to him how great I looked.
I cannot tell you this enough...plan your future for YOU and YOUR KIDS, not him. If he figures into the equation later (because of YOUR choice), fine and good. If not, fine and good.
