It's too bad that your graduation plan has to be changed because of adults acting like children. At this point, I would say, do what you think minimizes unpleasantness. I understand you not wanting drama. These adults seem poised to create it, given half a chance. It's your day.

Not to belabor the point, but the person who has the least claim on your consideration is Mom's boyfriend. If you've reached a decision that you think is best, then go with it and don't drive yourself nuts over this. However, this issue will probably come up again in the future. I'm not saying the boyfriend is responsible for anything. Your mom did what she wanted to do. But the boyfriend has no claim to be included in any family event at this point. You should be courteous to him, but he is not "family" - not yet. What the boyfriend wants should be the least of your concerns. What the boyfriend understands is unimportant. This is not a culture clash. Ex-husbands can feel hurt seeing a former wife with a new man. That is true in every culture on earth. A nice woman doesn't flaunt her new man at a family event that her ex-husband is attending. Your dad has to eventually adjust to reality. But he should be allowed a little time to make that adjustment. Your mom was setting up a situation with no consideration for the feelings of the father of her children. That's not nice. She's apt to do this again, so be prepared. Of course, none of this is your fault.