Let me float a thought - - - how do you know that they (at least some of them) are NOT standing up for you to him??? They just might be. Just not reporting back to you about it.
You said that two daughters asked to see the FB message and/or letter, and were appalled. You said: "Both daughters were speechless at the venom expressed and the nature of the accusations he made against me. One daughter burst into tears and said, "He's just saying what Dad always says about you! Dad's telling him those things. I have never read anything more terrible and untrue in my life!"
I would wager that these two daughters have in fact further discussed little brother's behavior between themselves, and have probably gone to bat for you with him. Probably unsuccessfully, since he is, I believe you said, and 18 year old - still in high school. There's not much getting through the thick skull of an 18 year old boy - but I imagine they have said to him something like - - "Hey, Tom, cut Mom some slack. She did her very best, and you're not being fair to her."
Now, being an 18 year old boy with, of course he doesn't get it. He might not for another 10 years or more - but that doesn't mean that they didn't TRY. They just wouldn't have mentioned it to you - especially since they didn't succeed in getting him to be fair.
Think about it - your friend who did speak up to immature son for you - she didn't mention it to you. Didn't come boasting to you about how noble she had been. She only talked to a third party, who was the one who told you about it.
Your daughters may very well have stood up to your son for you - they're just not bragging about it because they want it all to go away, and being in the middle is not comfortable for anyone.
Anyway - it was just a thought - so why not assume the other kids HAVE stood up to immature son for you - they just aren't discussing it with you. Just working on him and hoping he will grow up some day.
For all I know, our son HAS stood up to his wife on my husband's and my behalf (other thread) - he just doesn't tell us he has - because it would open a can of worms.
Go to the wedding, enjoy it, and assume everyone else there knows what sort of person your Ex really is, further assume that all your children except the youngest bone-headed one very much support you and stand up for you. I think it is not realistic to expect them to make a production of it in your presence, or to brag to you later about how noble they were. Perhaps they are quiet diplomats.
Best wishes.