I was thinking what Ilovecheese mentioned - who knows if DH's XW wants him at the smaller event? His AC may love the idea, but this is their mum's birthday - DD should have checked w/ her.
Just as DH should have consulted w/ you. Do you see a pattern here? Clearly, neither he nor his DD feel they need to consider anybody's wishes but their own. I know you've decided to attend both events, but, IMO, you need to have an overall talk w/ DH about this and let him know from now on, you expect to be consulted about any plans that will involve you. He'll argue that he "should" be free to do what he wants/see his kids/etc. But, IMO, the response to that is that you should have the same freedom and he takes that away from you when he makes plans for you w/o your having a say.
I'm not sure why you still feel like an "outsider," though, if you've worked at building a relationship w/ his AC all these years. Have your efforts not worked? Why are you left only w/ his X' DH? I understand your not wanting to converse w/ his XW, but how about his AC? If it were me, and I were still being treated like an outsider, I wouldn't go to the smaller event. But you've already decided you would, so yes, I agree w/ those who say put on a smile and be gracious. It will be over quickly.