The hardest part is making a decision. The back and forth, what to do, not to do, is a major strain, at least that is what I have found. I know you chose to stay back and not go, because you can't stand the thought of hurting your son. on the other hand, how does this effect your other children with you not going? Are they ok with this decision? Will they resent down the road that you didn't go because of son?
I have to agree with SWOT, in that I wish you son would tell you, "Mom, please go without me". That would be the honorable thing to do and it would show that he cares more about you then himself. It would be taking responsibility for the chaos his decisions caused the family. I am sorry husband gave in so easily and was just concerned about getting the refund vs. trying to sit down and talk about how to make everyone happy. I know my husband, if I said I wasn't going, he would have a fit. He would and probably say, if you are not going, then no one is going.
There are many issues I think going on in the family dynamics and I pray that they get resolved.
For the record, my opinion about son staying back is not a form of punishment, it's called accountability. I feel bad that your other children will be cheated out of memories of this once and life time vacation without your presence. BUT, you have to what you feel is right for you, for now. I am glad you came to a decision as it's been stressful on your mental and physical health.
I pray that your son is able to make a new life in being sober, stays off drugs and alcohol.