What you both say is very interesting. After recently experiencing how clueless my H is about how other people will respond==ie when he was utterly shocked that our adult children would be mad at him--I am finding it hard to believe he has any real thought as to the consequences of his words. My best guess is the C told him he had to be honest with the kids to diffuse some of their anger and he thinks this is the way to do it and then everything will be fine with the kids. Remember he is not intending on "outing" who the OW is as his mother likes her very much at the office and would be shocked.
Also, his family is the type that will gather the wagons round him as in their eyes family can do no wrong and his older sister did a similar thing about 27 years ago and left her H for a neighbor whom she married and then divorced about 17 years later too. His mother initially took first H side in A but then said she was wrong to do that. The interesting thing in the mix is my kids--grandma and family has never had to deal with adult children who dont have to come to holidays if they disapprove or OW is there next year.
I have a DB phone consult tommorrow with same C we met with in person. I am going to bounce this off her too. I have some time to decide. H has already said he would honor his word to me not to say anything but I can talk to him when he comes over to see our daughter in from Atlanta. Maybe I can have a nice TS day and leave early before he makes his announcement. Kids could stay a make sure he tells the truth--which will be hard for him.