Yikes. Here's the thing -- your boys didn't create this situation, they didn't ask for it, and they can't do anything to change it or fix it. They are 100% victims in what is going on with you and STBX.
With that being said, their needs have *GOT* to come first, before NGS issues, before standing up to W.
I *strongly* suggest you meet with a child and family therapist and talk about the dynamics that are going on right now and how to best take care of your boys' needs. It will be an excellent investment in time and money to do so.
Having that fight in front of the boys is horrifying, that is absolutely going to scar them. You can't influence or control what STBX does, but you are fully in control of your role in things, and you should not fight like that in front of your boys -- ever.
One of the things that I learned is that for the boys, this process is terribly destabilizing. They view the two of you as their rock, and when you can't get along and treat each other poorly in front of them, they can't trust you not to treat them poorly if they make you angry, and that will make them anxious. You're showing that your love is conditional, and you don't want them to worry about that.
Secondly, routine is important, it's something that will bring them comfort. If you can preserve some family traditions for some period of time it will make their transition much easier. If the birthday dinner is a tradition, you should suck it up and preserve it for his benefit.
In my case, we kept having family dinners once per week for six months after we were divorced, then tapered it down to twice per month, then once per month, and now only for special occasions.
It's true that you want to show the boys that you will stand up for yourself, and not tolerate bad behavior from W, but you also are responsible for protecting them, and sometimes those agendas are in conflict. I would avoid openly fighting with STBX in front of the boys at all costs. You can disagree, you can argue, but once emotions come up and things get unproductive it's time to shut it down. That's the model of behavior you want to teach, not getting snarky and lowering yourself to the lowest common denominator when you get upset.
So what's best for the boys?
-- Showing that although you two don't want to stay married, you both still respect each other -- Showing that although you two aren't agreeing on things lately, you can still discuss disagreements productively -- Showing that although W may antagonize you, you will not be provoked -- Showing that although you don't want to hang out with the STBX inlaws, you can be a man, show up for your son's birthday and be pleasant and civil with everyone for his benefit.
Demonstrating that you can show up and be pleasant and civil shows a lot more strength than avoiding the situation to somehow punish W for failing to consult with you. Really she's not going to care, its your son who will be punished.
I also don't understand why when W asked you where you were you gave her the "none of your business". What does that accomplish? It comes across as very passive aggressive and would infuriate anyone if it were a consistent behavior. That's not being strong and independent. Being strong and independent would be not caring how she feels about where you are and what you're doing, and if you don't care about her reaction, then it doesn't hurt to tell her where you are. Who cares? She may have a legitimate need to know from a parenting perspective, maybe she needs to go somewhere and needs to know when you'll be back. In any case, it doesn't matter because your whereabouts really weren't a secret. It's good to be mysterious, it's not good to be evasive.
You're antagonizing and provoking her with this "you don't need to know" stuff and I would drop that, to AS' point it's not going to get you where you need to go.
I love that you're working on sticking up for yourself, you just need to tune your approach.
Acc
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015