You don’t respond to her attacks. You don’t compliment an abusive woman sleeping with another man while married who actually told you , her husband, is her back up plan.
And your 7 year old son must have seen that communication in your marriage was fighting. So he chose the only way he knows how to get you to communicate .
And making telling him to behave at his mothers all the time will leave him with guilt .
Not much is going to change until you stop rationalizing and making excuses for your actions that are absolutely not helping you