I agree with most of the other posts so far. A couple of thoughts here:
First, this is a transition stage. As such, it is the highest anxiety, highest clash-y part.
It's also very important to stand your ground now, as you seem to agree with, because however things get set up now will soon become "the usual" going forward.
The good news is, all you have to do is get through this period without giving in AND as kindly, calmly and drama-free on your end, as possible. And it will soon all be settled, with the least scars possible for all concerned.
It's too bad this isn't what your future daughter-in-law hopes for and envisions. But it's simply not workable for you. We rarely get everything we want and this is your call to make, not hers or your son's. It seems they're pushing hard for things to go their (her) way right now. But that will stop when they catch on that it's simply not going to be.
I think it's nice, in a way, that they do want everyone included. After all, many young'uns would just want one or both sides of the family out of the way. So that's a little positive piece, anyway. Also, highly likely in my opinion that she wants you there as a buffer between herself and her drunken, obnoxious parents. She'll just have to find a healthier way to handle them then, if that's her real agenda.
I don't blame you at all for not wanting to establish that your holidays will be spent with drunks who get pushy and even aggressive. Good for you! And it's so much better to just keep your distance from the start, than have to deal with the inevitable ongoing drama these folks bring to the table.
And remember, your son and his fiance might be in their mid-twenties but they still see what you do as a possible model to follow. And you are modeling good behavior- not being pushed around and not tolerating drunken obnoxiousness. All while standing up for yourself as kindly as possible. It's quite possible they'll follow the lead you're setting, sooner or later.
Personally, on that note, I wouldn't lean too heavily on your social anxiety. That makes it sound like you agree that you are the oddball, while, in reality, they are the oddballs. I'd try to keep distortions that favor them and disfavor you out of it.
Just calmly and kindly repeat as necessary that you're used to and prefer smaller, quieter holidays. Ask them then if they'd rather come to your house before they meet with her parents, or the day before or after. At the same time, avoid any name calling or disrespect about her parents. Set the example to follow.
Don't get into any big discussions, explanations or arguments. Simply keep saying no thanks and asking which other time would work for the two of them. If it escalates, tell them you have to go but to let you know which time and date they prefer.
If they insist that they won't meet with you at all, calmly tell them that makes you very sad and you need to go right now. It is up to them too and they may have to learn a lesson or two the hard way but letting them push you around only teaches them that it is a workable way to deal with you.
I think if you just do this and keep the emotions turned down low but kind, this will likely soon settle down in your favor.
Best wishes. :)