My DH and I were not invited to SD18's graduation either. She wants gifts so she sent my dad a notice but I told him not to buy her anything, because she hasn't made any effort to stay in touch with him. She hasn't ever said thank you to him for the gifts he was kind enough to get her.

DH decided he was going to his daughters graduation without a "ticket." He pretended to be a security guard and they let him in. He watched her walk to get her diploma, took a pic, and left. SD refused to believe that he was there until he sent her a pic. I thought it was funny, but also sad for him. 

These Skids deserve whatever is coming to them. I will hope they turn out as better people but I am not holding my breath.

The day before she graduated, DH asked SD18 about going out to dinner after graduation. She told him no, that she already had plans with her mom and her boyfriend's family

Well it seems like he asked for her permission which gave her a choice to say yes or no. 
 

Stop giving them choices when they don't deserve them. 
 

"Look little Suzy....we've done ALOT to help you get where you are today and we are hosting a graduation party for you and our family so which date would be better may the 15 or May the 30th?" .... those are the only two choices there is no choice to say no here unless they are laid up in the hospital or some other serious emergency. 
 

Dont expect to be included in BMs hosted events. Host your own. 

My parents would have side-eyed me, ran multiple guilt trips, told the entire family, and probably gave me some negative consequence if I dare told them no they couldn't host a graduation celebration for me.
 

Unless parents are abusive or something then denying them this opportunity to do something nice/celebrate their kids during these once in a lifetime milestones is a pretty messed up thing to do  

Should I break my own rule, now that they're all adults? Tell them, diplomatically, that their mother is the problem, and they need to stop excluding their dad over HER drama? Or will it just make things worse?  

If his kids are so obtuse and inconsiderate to tell you all no they don't want to be with you all for a graduation celebration then I would not trust that they'd be emotionally intelligent enough to handle the truth about their mother. 
 

Don't waste your breath 

If your DH isn't so offended that he says nothing to them about their rude behaviors then he doesn't really care that much. 
 

You can not be more offended then he is.
 

This is is battle for him to figure out how to man up and tell his kids about themselves so they can correct their behavior. 
 

Instead of bring up their mother I'd question my spouse if I were you .... like "Bob are you okay? ... your children seemed to have disrespected you on multiple occasions (provide the facts) and you never said anything to them to allow them to correct themselves. Is this how you prefer to be treated by them?" 
 

Side note = a lot of BMs their spawns have levels of entitlement that are out of this world. They expect you to sit down, shut up, and be a walking ATM for their unlimited whims 

Ugghh....I know you can't put all the context in a short post but.....why would you all want to hang out with BM if she is full of drama as you say? Why can't BM say that she doesn't want to hang out with her ex? (that's pretty reasonable). Also if SD knows there will likely be drama, then she can choose to reduce the drama by only seeing one parent at a time. Why was your husband only asking the day before graduation about plans for the evening after? Why when SD was busy that evening didn't he say....well how about Wednesday night?

It sucks that her mother seems to be winning the popularity contest that is parenthood, and I am sure there are numerous examples where similar situations have happened. But I do think in this case your husband is expecting a bit much to be invited to last minuite to a celebration with his ex. 

I know the fantasy of "setting the record straight." DH has never bad-mouthed BM the way he could have, but he has told the skids the truth when they have hounded him over his "abandonment" of them--the false narrative fed them by their mother. 
There has been so much PAS in our situation that I likened it to a cult many years ago, and in many ways, once they entered adulthood, it became more cultlike than ever. 
It was and is useless for us to set the record straight with BM's minions.  Here, we have resigned ourselves to the fact that they are too far gone. 
But if DH goes before me, believe me, I'm setting the record straight with his daughter in writing. Setting the record straight is for me, not me expecting any change. 
If I were in your shoes, I would start holding back on what you all do for the skids. Perhaps start behaving the way you are treated. 

DH is absolutely not the favorite parent here either.

YSD18 just graduated. I was not invited to graduation so I did not attend (been involved with SDs for 11 years now). So no gift from me.

DH suggested to YSD, a few days before graduation, that they do a dinner for her at her favorite, and only restaurant, she'll go to. He never got an answer and only a shrug grad night. He was disappointed.

He's also not going to college move in day because "it would be awkward" according to BM. WTF. He's just letting them dictate again and being hurt again about it, in spite of me constantly telling him to not be so freaking passive！

I've got no answers for this but to recommend staying out of it except with your DH privately.

Is it possible that it's not just that BM is uncomfortable having you and your husband there, but perhaps it's uncomfortable for SD to have her parents together? I actually think that's a pretty standard sentiment for children whose parents had a contentious divorce. She could just be blaming it on BM because that's easier than being honest about her own feelings. But feel like your DH could've been more proactive about planning his own celebration for SD's graduation if it was really that important to him. 

I know my SDs would rather die than suffer through a meal with both of their parents. Fortunately that's fine with all the adults involved too and we've just learned to plan around each other's stuff. 

Speaking as a BM, there are situations where everyone must be together. These include weddings, funerals, and graduations. Things where there can be only one ceremony. As far as birthdays, holidays, dinners and parties to celebrate graduations, etc., i would rather have those separately.

When my son graduated high school, my ex threw a big bash the night before. I felt like i had to go, but after i went, i realized it would have been much nicer to have my own celebration with my son. Joint things are awkward. There was a reason we split up, many in fact, and one of the best parts of being divorced is not dealing with my ex unless i have to.

As far as the kids' feelings, of course they would be upset if one or both parents missed graduation. But they have both said they prefer to do holidays and all other things separately, or are ambivalent. I came out and asked once if one of them wanted to do a joint dinner for graduation and got a firm "no!" To them, one of the best parts of coming from a broken home (other than the lack of domestic violence and daily screaming) was double celebrations/double presents. 

While I understand the emotion of avoiding reality, it is not an effective way to raise CODs IMHO.

Reality is that things are not identical between mom's house and dad's house in a divorce or otherwise failed family situation.  In our case, DW was a single teen mom. She was never married to my SS's SpermDad.

Facts are far more effective than avoidance.  Facts. Are neither good nor bad. They are merely facts. They are not badmouthing.  But kids do need the facts and the quality side of the blended family equation needs to present the facts to the kids so the kids learn how to protect themselves from the shallow and polluted end of their failed family gene pool.

So, I would roll out the facts. SD is ostensibly an adult and she should be force fed the facts so she at least can't ignore BM's toxic bullshit without chosing to be self delusional.

While my SS did not have a deprived experience when on SpermLand visitation, we for sure did not mitigate the differences in his real life at home with us and the shit puddle his SpermClan splashed around in. 

Part of our reasoning in letting SS live that difference was that he saw how hard his mom and I worked on our educations, our careers, and how he lived due to that effort in comparision to how the SpermClan did things and how he lived when he was with them.

We adopted the facts based model when he was fairly young after he would come home from SpermClan visitation upset that we were starving his three younger also out of wedlock Spermidiot spawned half sibs by two other baby mamas by taking money from SpermGrandHag, etc, etc, etc...

So, we chose to counter that bullshit with the facts. We reviewed the CO with him explaining that we were not taking money from his younger sibs. The CO is the law. We have to follow the law.  It was all presented in an age appropriate manner at the time we had to counter their toxic bullshit.  As he got older, he would come home from visitation with questions. We would answer them factually without rancor.  Eventually when he came home after them playing some crap that did not pass his smell test, he would dive into our Custody/Visitation/Support files and do the research for himself.  When he gained confidence and knowledge to start calling their bullshit in real time while on visitation is when the dynamic started to shift and their toxicity was stopped in it's tracks by SS himself.

Interestingly, the calls to my DW at that point were always "How come he knows about that?" The answer was always "Because it is the truth. We do not lie to him. He knows the facts because he asks. Stop lying to him."

For sure I am team give the Skid the facts. Always. Do it in an age appropriate manner. The earlier they are introduced to the facts, the less toxic drama they get from the other side.

IMHO of course.

End result, our son is a viable self supporting adult, a man of character and standing in his profession, life, and community. His three younger Spermidiot spawned half sibs include spawn #2 who is on the dole, spawn #3 who is in prison, and spawn #4 who is not far behind the inmate.

Once SS made it clear that he would not support the SpermClan financially after he aged out from under the CO, they have had nearly zero contact with him.

I would go with "I'm sorry your mom has struggled to adjust to not  being married to your dad. But, it is not acceptable for you to turn your back on your dad and on me just because your mom has issues. Please do not do that anymore."

There's one right way and hundreds of wrong ways.  The child should be the important person.  They should spend time with both parents.  Both set of parents can just let things work out.  There should be a CO in place spelling out visitation, CS, medical,  ect.  
Its when there other SO involved, where things go badly.   Most SO don't want there SO talking to each other.  Lots of water under tge bridge.  The one they perform sex acts with.   Once you Fu** someone you don't go back as friends.  
'some many people , so many emotions, some many people fighting for control.   " If you love me, you will not talk to your ex". 
'People don't talk to people they were in relationship with.