I understand how you'd be tempted to send that letter but since you asked, I'd advise against give them anymore ammunition against you by putting it in a letter that can be dragged out years later. I'd advise that you just address each person, if and when the situation arises and I'd factor in your annoyance about all of this and tamp whatever you say, down a bit.
If you do send a letter, (although it's a stretch) you could thank them for their well intentioned advice and tell them that you think from this point forward, it would be better if you and (husbands name) work things out between you.