So sorry this is happening to you, op.
You need to stay stronger than ever over the next few weeks, so it's good that you have some counseling planned.
I say this, because the fact that it's all come out into the open has just made these pair of silly twunts realise that in the cold light of day they weren't the sexy characters out of a book with nothing to lose.
I mean Christian Grey wasn't a middle aged office drone with skid marks in his boxers was he?
I'm sure he fed her all kinds of lines about you not having sex with him, or enough sex with him, or the right kind of kinky fuckery (wtaf) that was in the book. I bet it would be a huge wake up call to her to know that he was actively having sex with both of you. So if she calls again I would tell her that he is free cycling your sex toys and that you are having STD checks because you were sexually active.
Now she's shitting herself because the bubble has well and truly burst, and her whole future lies in your hands. I bet the woman had no intention of leaving her dh for your dh.
And now your dh is left high and dry like a sad lonely tw*t. This is the danger time - she will be on course to reject him, and he will convince himself that he had every right to take back the reins as head of your family. He will then use any foem of emotional blackmail possible to create a pity party for himself, in the hope that you will welcome him back.
So keep those emails, if you are heading for a divorce you can use them, and make sure she thinks that you intend to name her in the proceedings. If not, keep rereading them to convince yourself you are doing the right thing here.
Your ds1 sounds wonderful, take care of him, and if you have the energy, try to lighten his heart a little of all this burden. X