MFIL,
You asked me for comments and I am sitting here trying to remember all I have read about your situation. You two have a child together, who has custody of the child?
Have you given an thought to separation, I mean legal separation?
At your age my inclination would be to say "goodbye" to her, but with a child that is never easy nor prudent.
Given that her family supports her having an affair, you two are still married right, I would like to suggest to you that the fruit has not fallen far from the tree.
I really don't know what to say to you, but I will offer you what your life looks like from someone almost twice your age.
From my end of the telescope you are a young MAN. Turning 30 is about where I think most people start to be really adults.
If you live to be my age and the stats say you have a high probability of doing so, your child will be what...older than you. You will have worked for over 30 years from where you are now. You will work at least another 5-10 years, statistically, assuming you don't hit the lottery or you don't become the CEO of a big corporation.

You will have roughly another 30 years to live when you hit my age.
Why am I telling you all of this? I want you to gain some perspective. I want you to realize that if you divorced right now and did not remarry until you were 40, that you can still be looking at 50 more years of marriage. If you are going to be married that long, you need to have chosen wisely. You will need support, guidance, love, and kindness from your spouse, and she will need the same from you. You will need to find an inner peace with her, and for these things Viagra does not help. You will need to find comfort with her. Your child should like/love and respect her, although she is not her mother.
Your W caused this marriage to end. Given the requirements that I have set forth for a W that you will live with for roughly 50 years, does your W meet those requirements? I don't think so.
She is in her 30's and what you see is what you get at this point. Oh she may mature some, she may actually experience what you have experienced and realize what she has done, but it will be too late.
MFIL, you need to start to look long term at your life, your goals, your talents and start to make plans. If your W comes back into your life in time, fine. If not, count yourself blessed. Blessed not becuase the marriage failed but blessed that you have time to really have a great life. You my friend have a future. Plan for it.
In my 20's, I was engaged to the "love of my life", she cheated on me, wanted the party life, and we became "disengaged". A few years later, she was murdered by one of her boyfriends. Meanwhile, I am in my 60's have a wonderful W, 3 kids, and many friends. I still think of my fiance from time to time, and I grieve for her parents if they are still alive, but I know they were devastated by what happened to their daughter. They were very good people. Could we have had a good life? I would like to think so, but looking back I don't think my life could have been as it has turned out. She had many qualities that I loved, but she did not have the qualities that I needed to pursue my life and have a family.
What happened to me then is part of why I stay on this post, and the other part...I realize I have been blessed in many ways in my life, and it gives me great pleasure to help other people although it is very little.
MFIL, I don't know what your future is. I cannot tell you what the "right" thing to do is, given that you, your W, all of us have free will. But, I can tell you that you not her may be the lucky one right now. I like your chances far better than I like your W's. Why? Your heart is in the right place, you value important things.
My suggestion...take a chance on life and be the best person you can be.
I know I haven't helped you with your problem, but I hope that some day you can look back and say "Yup Ole JL, put his money on the right horse." That horse would be you.
God Bless,
JL
PS: Your situation is not unique, it is just that the search function doesn't seem to work well and I don't remember all of the names. I do think that more than a few posters here have had their spouse move from one OP to another. And you are wrong about OM#2. Your W is still married and he is not helping your marriage.