Hi D..
This sounds like two similar issues that maybe it will help you to separate out a little more.
On the one hand, letting your parents have their journey. ie supporting your mother in finding the place in herself that needs your father to be a certain way toward her; and supporting her in uncovering the deeper meanings this may have for her in regards to how she treats herself and those around her; and supporting her in finding a way to provide herself with the loving care that she wants from another so that she's not dependent on others to meet her needs, so that she is not a victim. Certainly parts of her may feel like she had no choice in the matter of separation and divorce from your father, and those are certainly valid feelings and valid perspectives, but deeper down, what gift are they bringing for her that is a greater way of meeting her needs, perhaps with less "cost", less "price" that she must conform to or "pay out" in her interactions and relationships with others.
She may or may not be willing to open herself to these uncomfortable reorganizing considerations, and if she is willing, it could take a weekend workshop to shift her perspective and it could take several years of counseling. It just depends on how she wants to do it.
It might be helpful for her to ask herself, what would really be most joyful for her in this regard, now that things are as they are.
On some initial level, perhaps it feels more joyful to her to get some revenge, and that's a rather important and significant step toward feeling better if we're in a place of depression. Anger and revenge feel better than depression, but we don't want to stay there for the rest of our lives because there are so so many other places that feel much much better than revenge. I would even say that revenge feels better than guilt, but then after revenge, taking some responsibility can feel better also, because we've gotten some juice flowing again and we often don't feel as wounded after some feeling of blame and self-righteousness.
It's important to relate to her where she is, not to only an ideal end point of how she should be, because it's just too far to jump from being angry to being totally loving and joyous toward everyone around us, and it sounds like she's not ready to be the ideal that you might know is possible for her and in relation to your family.
Things must become what they are before they can change.
It seems to me that this is the second part of the issue for you: of how to relate and how to orchestrate family gatherings and such.
But not just family, how do you relate to her about this.
Judging her as "an immature teenager" may be an accurate and appropriate assessment for your understanding of what is happening, but likely she won't be any more equipped to make a better situation out of it by you holding that opinion than a teenager would who feels stuck in a situation they dislike and that they feel very strongly about.
When we feel strongly about disliking something, we are defending an ideal, and we're not being willing to make the best of what is. Like a teenager who's heart has been broken by a situation that is so far from their desire for themselves, she is confused and unwilliing to let go of her dream. It's not that she needs to let go of the dream, for our dreams give us life, but she needs to find a way to feel better within herself and about herself and also about other's decisions so that she can begin to find a way to allow her dreams and the longings of her heart begin to take shape.
It's easy to let love in when we feel worthy of love and willing to love others regardless of their decisions. And when we're upset, that somehow colors everything we hear from almost everyone so that we become defensive toward almost any imput because we don't want to be made wrong.
When she is feeling defensive, it is probably most helpful for her (and the overall situation) for her to feel heard in her opinions and feelings and that's all.
No analyzing, no conclusions, no "well if that were so, then ..." or "if you were to do that then ..." because she won't be able to hear it when she needs so so much just to be heard herself in her feeling of being wronged. You don't have to make it right or wrong. You don't even have to agree with her. Just tell her what you're hearing her say and ask her if that is what she's saying so she hears it back from you. Don't be trite with it. You must be genuine, and it may take time.
Of course, this all depends on how close your relationship with her is and how willing she is to hear about it.
It may help to just lay some ground rules, perhaps even written (by both of you--and maybe your dad too) and signed, so that everyone is aware and accountable in a different way. This may be quite a trigger for her since she may feel that he didn't honor what was written before so why should she now, etc, but it may help.
My own parents have been divorced for about 10 years. They raised phenomenal children and my mother doesn't want anything to do with my dad anymore. They were both at my brother's wedding two years ago, and there was no strain on the event, but I don't think they talked the whole time. I didn't see it, though I wasn't really watching. I'm sure if they did, it was cordial. They're essentially strangers. There is too much pain on both sides for too many reasons to unravel here and for too many reasons for either of them to be interested in attempting to untangle together. This is where they are and I feel the best I can do is to love them both and not to push them at all with an idea that I have about how they should relate.
They probably will never come to an openhearted, easy, verbal agreement, but they each have come to a certain peace within themselves according to their beliefs. My father has remarried and my mother has been in a long term relationship for years (which ended about a year ago) and now she's really discovering who she is, outside of other company. Quite a big journey for her and I'm really delighted for her in it.
If I could encourage your father at all, I would encourage him to not "take on" your mother's display of disapproval.
It's the old saying about a show-off (not to be disrespectful in any way toward your mother), but when someone wants attention and we dont' give it to them, they stop acting out eventually.
I would encourage him to let it be like water off a duck's back.
If he makes a big enough deal about it to not come to other functions (that are important to you) that is doing at least two things. It's playing into what your mother wants, which is probably not good for anyone concerned. And he's making a big deal about it too, even though he thinks he's trying to mitigate a big deal being made, he's acting out in another way and that makes ripples just as large as what she did openly.
It's really really essential that we create a feeling of allowance for other's feelings, and not make them wrong or inappropriate. This is essential in our family circles for us to be able to function as a family.
It's difficult when the parents dont' know what to do, because parents are supposed to know what to do. That's their role, but it is up to us as children to remind them, now that we're old enough that it is sometimes more important, even if something makes us uncomfortable, that we continue to contribute to a spirit of well-being and not begin to co-create a feeling of disease or to ostracize and isolate one individual or one way of being.
This is what I would encourage your father to bring: a continuous spirit of well-being regardless of the displeasure expressed by others, for even an expression of displeasure is allowed in the overall expression of a healthy environment.
I've run out of time to respond, and hopefully this is helpful both to you and to the whole situation.
Be Well
P.