Ok, I'm warning you up front, I'm going to be harsh, but I believe it is for your own good.
WTF are you thinking? Why would you even consider sending this? Stop feeling and start thinking! If you will start using your head you will see why this is such a big mistake.
First off you say that you are doing this because you are trying to guess what the OM would want and feel he would want to know this. That is a bunch if BS.
WHY? Because the first thing he will do is ask W if this is true. He will think that you are making up a bunch of lies to try and get him to back off. Reread your letter. It sounds like one big pity party. You can't sleep, you miss her sooo much, it's killing your kids,bla, bla, bla.... He doesn't give a shoot about that. Never did and never will.
Another thing, you say you think he is a decent guy. More BS. How can you honestly believe that. This guy is a selfish arse. If he was a decent guy he would have never gotten involved with a married woman. I'm not the most decent guy in the world, but when a married gal that I care about deeply, got drunk and asked me to come over to her house I turned her down (her H was out of town). Why, because I cared and didn't want her to end up like you and your family. That's what a bit of decency is. Not what the OM is.
I also like how you say that him and his son deserve more. That's a nice play on words to try and make him think that you care. Do you know again what he is really thinking? He is thinking how could you know what is good for me. How do you know that what your W is telling me is untrue. He will think this is just more self pity and lies from you trying to make him go away. Because he knows how much your W really loves him.
You know what, he may not talk to W, but if I was a scumbag like him it would give me the perfect opportunity to put the final nail in your R coffin. I mean if I was him, I would show the W your letter because I know it would piss her off and drive her further away from you. What a perfect opportunity.
Really think about what I said. File the letter away and look at it again in 6 months. You'll probably look at it and wonder what were you ever thinking to have written this.
"Our life is what our thoughts make it."
Marcus Aurelius