Ok, don't kill me for saying this.
But in your FI's defense, at least he has been listening to you and the problems you are having with his mother. And he took a step to try and put ALL (lol) the problems on the table. Granted, it was probably in the wrong way,lol. But I think he took your frustration and feelings into consideration and since he is the 'middle-man' here, I think he was trying to make you happy.
Lol, now the conversation you guys had when he found out he was busted, now thats funny. lol. I do agree he could have at least given you a heads up about it, that way you would have either a) monitored the caller id and didnt pick up, lol or b)would have had time to prepare your response.
With all that said, I hope everything gets worked out.