I went through the same thing with my parents 10 years ago. It takes time - a LOT of time - for it to get better. My parents can now be in the same room together, but they aren't friends. They suck it up and deal with each other for the grandkids brithdays now, but it took time.
And I know you don't want to hear this, but your traditions are going to have to change. You can't have everyone at everything or expect that inlaws/cousins/parents are always going accomodate your schedule. You have been very lucky up to this point to have such wonderful people in your life that they celebrate holidays on different days so that you can have your holiday the way you want it. And celebrating with your ex - wow, kudos to you and him for your maturity and foresight. Unfortunately, that tends to be the exception rather than the rule when it comes to divorces.
While my parents were in the stage that yours are, I had them TAKE TURNS and told them that was what I was going to do. I kept track of who got to have first right of refusal. For my son's 1st birthday, my dad had first dibs. He decided to come, so the decision to attend was my mom's. I told her that it was HER CHOICE to come or not. She choose not to come and celebrate the birthday later in the day, by coming for our regular dinner - nothing special - I did NOT throw a special party just for her. The second she said anything about missing out, I reminded her that it was her choice to do that, not mine.
The first time that she had first choice and my dad RSVP'ed that he would like to be there too, my mom threw a fit. It was her turn (I swear she sounded just like my 3 year old...)! I talked to my dad about it and he made a great point: This was between them, not me. Me trying to get in the middle and make everyone happy wasn't going to work and was only going to make everyone miserable and stressed.
So here's what we all came up with: The person whose turn it was had first dibs on the event. If the second person wanted to come, knowing that it wasn't their turn, they had to call the first person and work it out. I was not a part of it.
You have to have the willpower to do this and not get in the middle of it - your mom will try to suck you in, trust me. Call it "tough love"! When my mom did try to pull me in a couple of times. I had to say, "no, talk to dad, not me, if you need to work this out."
It's hard not to get pulled into the drama, but just say no! You can do it!