I am so proud of you! (Not in a condescending way). My dad and my exA are narcissists and people view them like golden boys. They are not. ExA is finally showing himself as his drinking and irrational behavior grows. He was never kept in check and it spirals out of control without some sort of accountability. It really sounds like your exah is spiraling too.
I think your assessment about him choosing a weaker victim because you are not there/allowing him to abuse you is totally correct. My dad did/does that too. Narcissists are NO picnic to deal with and the least amount of contact the better. Supervised visits with the girls sound like what might happen. My mother (since my dad had custody) did something threatening towards me when I was about 10? and she had supervised visits with us from then on. BTW, She BLAMED me in private so expect your exah to have one personality with the court and one with your girls alone.
That is great you have an open dialogue with them. Between therapy and talking with you, they are going to be okay. You really did the right thing by leaving him, I hope you see that. No matter the trouble it is causing right now, being trapped in the chaos and growing up thinking that is normal would be much much worse for you and your girls.
Hugs