hopefulinnc,
Heart felt sympathy for your situation!
As far as advice, here it goes:
Of course your children must be told.
In fact, The sooner the better.
I understand your want to protect them. (Admirable!!)
Fortunately, They are all grown adults. (Not that you necessarily see them that way).
Please Don't sacrifice short term gain (avoidence), for long term pain (D).
To start:
First, perhaps everyone knowing can exert some added influence on your wife to come out of her fantasy.
At this point you need any and all allies you can muster to your cause. (not saying the children need to choose sides........just that they can be additional voices of reason,.... that is showing how wrong and unreal the A is).
Telling NOW lets You and the children work on her BEFORE she leaves the house.
NO guarantee to work, but if you do nothing she is guaranteed to leave.
YOU have nothing to loose on that one.
As far as How to tell them:
You should do it separately. (IMO) You and your W are NOT on the same page with this decision. Do you really see that family meeting going well?
That is with her trying to tell how she's found her soul mate and you trying to explain your pain and devastation? Yiikes!
I don't see how you can put on a "happy" face or any kind of united front when explaining this situation to your kids.
YOUR points of view on this are just too far apart. For all your good intentions the situation would deteriorate very quickly.
Individually, They can each talk to you, getting both sides. But only with one of you at a time.
The benefit is that This will let you both be able to fully express YOUR views, without having the pressure of "refuting" the other's position. Since they must know , let them get the FULL picture. Unfortunately this won't happen if you try to do this together . Sorry it just won't.
This information is going to be hard enough to digest. Don't put them in the position of having to see and hear the two of you at odds with each other, the very FIRST Time they hear it. Down the line, There can be plenty of other opportunities to sit down and discuss this all together. But make that a later time, when they've had a chance to process the reality of their mother's choices.
Also, Doing it one at a time will further protect them from the full force of BOTH yours & your W's hurt and anger. Of course they are going to see some of it from each of you. However, you'll both be better able to keep control of your emotions if your not doing it there together.
Next, I'm sure since this must be done, you both want to make sure you are heard!
To continue, Since you are the one with the problem of her leaving and having an A, I believe you should go first. (After all your W wants to delay this as long as possible, right?)
Indeed, Then after it is out they can go and hear your W's side. They are going to find out at some point. Might as well be now, thereby gaining you 2 immediate benefits
1). As stated before, you get more people to help you strive to end the A. I'm sure they will not agree with your W that the A is the way to go. Get them in your corner.
2). You get more people whom can give you personal support and comfort. At times like this you need all you can get. Especially family.
In addition, this will be one less burden for you to carry. (having to hide this awful secret from them).
Any time you can get rid of any more stress or problems in your life that is a great thing.
Furthermore, your NOT being a coconspirator to her A can be liberating in and of itself.
If she is set in this course, you can't make her stay.
But you can stop being an enabler. Stop helping to make this as easy & as painless as possible for her. OUT her now.
Ideally, Her A should have been revealed a long time ago. Even so, Everyone is going to know once she is gone anyway (especially IF it does go to a D). Stop protecting her secrets. Do not wait till August. I see no long term benefit to YOU, to go along with her demands.
Personally, I would be surprised if one or both of the boys doesn't already know something about all this. Kids DO talk. Especially about their parents. And these are not really young children here. They are all adults. They can and will handle it.
OF course they are not going to like what they hear. Perfectly understandable. However, since they are going to know at some point, doing it NOW seems to be the best option for you.
NOT pleasant to do? Agreed! Very uncomfortable? YES! However, it is a necessary evil.
With that said,
I respect whatever decision you decide is best for you and your M.
Wishing you success!
Take care