Congratulations on your beautiful son! And my sympathy to you for your spouse's choice.
I've read this current thread and struggled on what to say. So here it is..
The ring? Put it away for a year if and when the divorce is finalized, then worry about it. But I'd stop wearing it.
Your divorcing spouse has made his choice, 'heart over brain'. Now it's your turn, but following the opposite parameters, 'brain over heart'. Step back and be realistic about your situation. He's involved with another woman, has been. He's blowing you off emotionally while still taking care of the physical upkeep. You're paying almost the entire mortgage. Is this a guy looking for reconciliation?
Have you seen a lawyer to know your legal rights? That is always a prudent step. It doesn't mean you've closed up hope for your marriage, but you need to know what to expect financially and with custody now and in the future.
One woman I know was successful in her marriage after she found out about her husband's affair with a much younger woman. She consulted with a lawyer, then told her spouse exactly how much he'd lose (tons) if he went forward with his affair. They're still together after 50 years. Sometimes it takes a firm whack in the head to get their attention to be able to do the work.
It sounds like you gave him firm boundaries when you discovered the affair. Decide what works for you, not him. If he won't meet your objectives, then move forward with the divorce. Take control of the situation.
Stop trying to understand him (re: the letter) in passive/aggressive terms. State your needs, your boundaries and the consequences. Reconciling while your spouse is embroiled in a passionate affair is next to impossible. He's most likely to respond to an either/or choice.. where the little brain meets the big brain.
DBing does not mean sacrificing yourself. One person said it's a means to stop a divorce. And if that doesn't work, it's to do what's right for you, your child. Not what works for him.
You're a strong woman. Flex your muscles, drop the rope and move the divorce along. You have nothing to lose, aside from twirling on an emotional hook, tugged by his whims.