Regarding your S's grades, the D may have been the trigger. Many kids go through divorce without blowing up their grades, and many teens blow up their grades without a divorce. I hope you and your XW are able to work--either independently or together--to discover why your son's choosing this way to act out and what can be done to modify his behavior. At 13 there probably are at least some carrots and sticks (e.g., screen time) that can be effectively applied to him.
I'm so sorry you went through that experience with your D. I initiated my D just after New Years' and we did Easter together, so, fortunately, my kids were well-adjusted when the next Christmas rolled around. It sounds like a really wretched way to spend Christmas Eve.
