Ugg, how sad. You've been given a number of good suggestions, for instance broadening your line of support, per involving the school, also children's physicians, etc., anyone and everyone you can think of to be made aware of the situation and document, document, document, so you have independent reporting & opinions.
I also think you should have your attorney duplicate in his own words and by letter, send to her what you expressed by email. You can say the atty has been informed, but until she sees same, she will deny it in her head, thus not having the impact you hoped for.
I'm just wondering if you have children's advocates in your state. They weren't around for my divorce, but I do know states have them, either voluntarily or mandatory at some stage of the divorce process. The reason I mention this is that you have to be very careful how you question & interview children, essentially letting the subject matter free form from their mouths. A good case in point was a children's day care center that made massive press probably at least a decade ago. Seems the parents had been doing a lot of questioning of their children, once an accusation had been made & the accusations continued on, even including devil worship, besides sexual molestations of the worst kind. In the end, after much investigation, it was all proven false, essentially caused by overzealous (but understandable) actions by the parents & children wanting to please their parents with "correct" answers.
In your case, at the least, you know your son has his own agenda...that Xbox, hehe. We are around our children so much, it's hard to remember they don't have the same critical thinking skills as adults, yet they can speak the same language. I have no doubt he was hurt when he was lifted. My dad also did that move a couple of times and it hurt like hell, kind of like a friction rub. But my dad didn't mean to hurt me intentionally, just lift me up. So I'm not defending POS, just putting it out there that it might not have been intentional. And those noises from the bedroom "could" have been sex & dirty talk, ya know? Either way, you did the right thing by putting her on notice, I too, hold to the belief that only the natural parents should commence discipline. Sadly, in my one son's case, his stepmother caused a lot of damage with her words, something he was very quiet about & something I didn't hear about until much later. I ended up hearing about it after he got married from his wife.
Anyways, good on you, her response is predictable, I again encourage a letter stating previous from your attorney. Best wishes.