Asitis, you are right. I will try not to go there.
And I did take your advice last night and wrote him a letter. Of course I got no response. I kept it short, basically I said "thank you for comforting me while I was distraught, I believe and accept your apology for having the A", and then I said "I am still struggling with the lies, coverup, and ongoing justifications and deception. I do believe you that it is over, however, the continuing cover up and poor boundaries are making it hard for me to move on. Continued friendly contact and social media connections are sending mixed messages to me, to OW, and to your self. Please consider setting clear boundaries such as no contact. And please be honest with me about your interactions with OW.
Of course not direct quotes, but I kept it short and to the point and reasonable because I am sure it will be read to father-in-law, IC, and possibly OW depending on how much he really is still speaking with her.