As for the lawyer talk....How you go about it is all about you. For me personally, going the adultery route just wasn't right. I could have easily pursued it, but there was really so little to gain and so much more to lose (like co-parenting).
As for the letter and your last statements....Actions speak louder than words. Facts overrule theories. The fact is the other woman is very much in the picture. Until she is gone....everything else is cake eating.
When your mind set changes from this is how I deserve to be treated....To this is how I demand to be treated, you will have taken the last step.