I know this is a painful situation, Floria. As a mom, myself, I realize it must hurt you and dh a lot to see 2 of your dss estranged like this.
But I'm glad you've just decided to accept it and move on. That's the wisest course, imo. Trying to change things might just get you and dh estranged, yourselves, from one ds or both.
Reading your op, this jumped out at me:
"My husband has approached the son who feels most aggrieved son on many occasions in an attempt to broker some peace but each time this has been violently rejected. We accept this (we have no choice). The other son has never been approached about this."
Why did dh only approach the "more aggrieved son?" Did he ask him to put his hurt and anger aside, "forgive and forget" for the sake of the family? Even though your other son hasn't apologized? I can understand why that didn't go down well with ds. He probably felt as if he were being told his pain didn't matter. I understand what dh was trying to do, but I also see why it didn't work.
This got my attention, too:
"My husband had a most miserable 70th birthday lunch this year with two thirds of the family. There is always one family missing at what should be happy family occasions and the older we get the more painful and sad we become."
I'm a little confused. I thought you said you have 2 separate celebrations for everything, but now it sounds as if you just have one, with one family always "missing." If there were 2 celebrations for dh's birthday, why was he "miserable" when one family wasn't there? He knew he would see them at the second celebration. What kind of message did it send to the other 2 families that their dad/gf couldn't be happy with just them? Imo, you need to try more to focus on the ones who are there and less on those who are absent/coming another day.
I think it's great that you're going away for your Golden. You'll avoid the 2-celebration issue and give yourselves a wonderful gift, as well. But I also think you need to accept that the dual celebrations are likely to go on for a while. As others have said, many families do this, anyhow, for various reasons. Keep them simple (less work), separate and enjoyable.