Dink, after thinking about all of this, your kids are grown. So I think, in my opinion, it is less important for you tell the kids together. I would almost just cancel the meeting with her. And tell her you will handle telling the kids. Especially since this is the second time in 3 years.

Just when you get a chance tell the kids individually "Your mom and I are getting a divorce, and this time it is really happening." No need to tell them why ("We've just grown apart" is enough.) Certainly do not throw her under the bus by outing her PA. Trust me, that will all come out eventually. But the advice we normally give her is for minor children that will be now in a split home, being handed off between the parents. That is not your situation.

I'll open it up for others to chime in, but since your situation is involving grown children I think there is no need for the meeting to discuss, and it is just a bad idea in general.

M(53), W(54),D(19)
M-23, T-25 Bomb Drop - Dec.23, 2017
Ring and Piecing since March 2018