You know, something doesn't sit right with me about all of this.
When you are sending an apology letter, I believe that you truly should only address the people that you are apologizing too... ie the inlaws. I believe that it's no one else's business, or concern, because honestly, what others think doesn't matter and they won't understand it anyway.
So, what am I saying... people will hold grudges against you for things that they perceive to be wrong, despite the fact that THEY weren't the ones being wronged. It's their impression of the events and no matter what you try to prove to them, they won't get the message because they have this lens over you and are only going to look at you through their lens, and not a wide scope lens.
I think the apology letters are a fabulous and responsible thing to do, but I don't think you should ever feel that you have to explain your actions to anyone else that is not a party to receiving the apology letter. They will think what they want, regardless.